.github/workflows: fstests: use v6.15 for testing

We use Linus's master by default for CI. However, there's currently a
slab crash that prevents CI runs. Set the linux reference to v6.15 for
now.

EFI stub: Loaded initrd from LINUX_EFI_INITRD_MEDIA_GUID device path
[    0.000000] Linux version 6.16.0-rc2-g11313e2f7812
(dagomez@bgt140507bm04) (gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU
Binutils for Debian) 2.44) #4 SMP PREEMPT_DYNAMIC Fri Jun 20 11:19:59
PDT 2025
[    0.000000] Command line:
BOOT_IMAGE=/boot/vmlinuz-6.16.0-rc2-g11313e2f7812
root=PARTUUID=55d2b742-be5e-4c47-a4cc-5db4bbee0cde ro console=tty0
console=tty1 console=ttyS0,115200n8 console=ttyS0
{...}
[  OK  ] Started systemd-timesyncd.service - Network Time Synchronization.
[  OK  ] Reached target time-set.target - System Time Set.
[  OK  ] Started systemd-udevd.service - Ru…anager for Device Events and Files.
         Startin[    6.963883] ------------[ cut here ]------------
g syste[    6.964779] kernel BUG at mm/slub.c:546!
md-ne[ t   w6.9o66r69k9d] .Oops: sinevarlvid opcode: 0000 [#1] SMP PTI
[    6.968417] CPU: 1 UID: 0 PID: 384 Comm: systemd-udevd Not tainted 6.16.0-rc2-g11313e2f7812 #4 PREEMPT(full)
[    6.970443] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 2025.02-6 04/08/2025
[    6.972031] RIP: 0010:allocate_slab+0x26c/0x4c0
[    6.972919] Code: cd b9 00 00 00 00 48 0f 43 e9 48 8b 4c 24 08 48 39
ce 73 df 4c 01 e6 48 89 df e8 ef c0 ff ff 8b 73 28 4c 01 f6 4c 39 f0 75
8e <0f> 0b f6 43 08 04 0f 84 c7 fe ff ff f6 c3 01 0f 85 f2 01 00 00 49
[    6.976360] RSP: 0018:ffffa4878124fbb8 EFLAGS: 00010246
[    6.977340] RAX: ffff8fb84a538000 RBX: ffff8fb84019ca00 RCX: 0000000000007bc0
[    6.978729] RDX: ffffffffbd798ad8 RSI: ffff8fb84a538820 RDI: ffff8fb84a538008
[   i 6.c9e7870[]0 RmBP:  -00 00N0e00t00w0o00r000a R08: ffff8fb8bfff8a80
R09: 0000000000000028
[    6.978732] R10: ffffffffbd772748 R11: 0000000000000006 R12: ffff8fb84a538000
[    6.978733] R13: 0000000000000001 R14: ffff8fb84a538000 R15: ffffd8a6c4294e00
[    6.978735] FS:  00007f3272844980(0000) GS:ffff8fb8fe4ff000(0000)
knlGS:0000000000000000
[    6.978737] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    6.978739] CR2: 00005631b138a050 CR3: 0000000124afe003 CR4: 0000000000770ef0
[    6.978743] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[    6.978744] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[    6.978746] PKRU: 55555554
[    6.978747] Call Trace:
[    6.978749]  <TASK>
[    6.978752]  ___slab_alloc+0x92f/0xe50
[    6.978756]  ? copy_fs_struct+0x22/0xa0
[    6.978762]  ? copy_process+0x918/0x1c80
[    6.996149]  ? __do_sys_clone+0x65/0x90
[    6.996909]  ? do_syscall_64+0x50/0x1e0
[    6.997776]  ? obj_cgroup_charge_account+0x9a/0x280
[    6.998833]  ? copy_process+0x918/0x1c80
[    6.999657]  __slab_alloc.isra.0+0x52/0xa0
[    7.000568]  ? copy_process+0x918/0x1c80
[    7.001415]  kmem_cache_alloc_noprof+0x1bc/0x3d0
[    7.002421]  copy_process+0x918/0x1c80
[    7.003235]  kernel_clone+0x94/0x450
[    7.003983]  __do_sys_clone+0x65/0x90
[    7.004796]  do_syscall_64+0x50/0x1e0
r    7k.0056 14C] o nentry_SYSfCiALL_64_afgter_hwframe+0x76/0x7eu
 a[   t 7i.o00n73.84] RIP: 0033:0x7f32722e7202
[    7.008448] Code: 89 e7 e8 71 3b f6 ff 45 31 c0 31 d2 31 f6 64 48 8b
04 25 10 00 00 00 bf 11 00 20 01 4c 8d 90 d0 02 00 00 b8 38 00 00 00 0f
05 <48> 3d 00 f0 ff ff 77 5e 89 c5 85 c0 75 31 64 48 8b 04 25 10 00 00
[    7.012080] RSP: 002b:00007ffc38d31c60 EFLAGS: 00000246 ORIG_RAX: 0000000000000038
[    7.013529] RAX: ffffffffffffffda RBX: 00007ffc38d31c60 RCX: 00007f32722e7202
[    7.014861] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000001200011
[    7.016312] RBP: 0000000000000007 R08: 0000000000000000 R09: 00007ffc38d31f3c
[    7.017729] R10: 00007f3272844c50 R11: 0000000000000246 R12: 00007ffc38d31dd0
[    7.019284] R13: 0000000000000002 R14: 0000000000000000 R15: 00007ffc38d31e50
[    7.020869]  </TASK>
[    7.021392] Modules linked in: drm nfnetlink vsock_loopback
vmw_vsock_virtio_transport_common vsock autofs4 ext4 crc16 mbcache jbd2
btrfs blake2b_generic raid10 raid456 async_raid6_recov async_memcpy
async_pq async_xor async_tx xor raid6_pq raid1 raid0 md_mod virtio_net
net_failover failover virtio_blk psmouse serio_raw virtio_pci
virtio_pci_legacy_dev virtio_pci_modern_dev virtio virtio_ring efivarfs
..
[    7.028783] ---[ end trace 0000000000000000 ]---

Signed-off-by: Daniel Gomez <da.gomez@samsung.com>

Author: dagomez137

.github/workflows/fstests.yml

@@ -40,6 +40,7 @@ jobs:
           SHORT_PREFIX="$(echo ${KDEVOPS_TREE_REF:0:12})"
           make KDEVOPS_HOSTS_PREFIX="$SHORT_PREFIX" \
           ANSIBLE_CFG_CALLBACK_PLUGIN="debug" \
+          LINUX_TREE_REF="v6.15" \
           defconfig-xfs_reflink_4k
 
       - name: Run kdevops make