Don't mount the target file system if 'canmount' is not set to 'on'

[pvalkone]

I have a pool (tank) with the following filesets, which I'm trying to replicate to another pool (backup) on a remote host (espoo):

$ zfs get -t filesystem -r canmount tank
tank                  canmount  on        default
tank/media            canmount  on        default
tank/scratch          canmount  on        default
tank/timemachine      canmount  on        default
tank/x220             canmount  off       local
tank/x220/zroot       canmount  noauto    local
tank/x220/zroot/home  canmount  noauto    local
tank/x220/zroot/root  canmount  noauto    local

By default, ZFS will try to mount the target file system associated with the received stream. If the canmount property is set to any other value than on (e.g. noauto), mounting will fail and the service will enter maintenance state:

tank/x220/zroot:
newest local snapshot: zfs-auto-snap_daily-2015-04-15-2100
newest remote snapshot: zfs-auto-snap_daily-2015-04-14-0854
[...]
receiving incremental stream of tank/x220/zroot/root@zfs-auto-snap_daily-2015-04-15-2100 into backup/x220/zroot/root@zfs-auto-snap_daily-2015-04-15-2100
cannot mount 'backup/x220/zroot': Insufficient privileges
received 179KB stream in 8 seconds (22.4KB/sec)
Error sending snapshot.

This pull request adds a check for the canmount property, and if it's set to a value other than on, the -u parameter is passed to zfs recv:

       -u

           File  system  that  is associated with the received stream is not          
           mounted.

Example dry-run output:

tank/media:
newest local snapshot: zfs-auto-snap_daily-2015-04-21-03h10
newest remote snapshot: zfs-auto-snap_daily-2015-04-20-03h10
would run:  /sbin/zfs send -R -I zfs-auto-snap_daily-2015-04-20-03h10 tank/media@zfs-auto-snap_daily-2015-04-21-03h10 |
  ssh zfsbak@espoo /sbin/zfs recv -v -d -F backup
[...]
tank/x220/zroot:
newest local snapshot: zfs-auto-snap_daily-2015-04-20-2100
newest remote snapshot: zfs-auto-snap_daily-2015-04-19-2146
would run:  /sbin/zfs send -R -I zfs-auto-snap_daily-2015-04-19-2146 tank/x220/zroot@zfs-auto-snap_daily-2015-04-20-2100 |
  ssh zfsbak@espoo /sbin/zfs recv -v -d -u -F backup