dma-buf: Implement DMA_BUF_IOCTL_{EXPORT,IMPORT}_SYNC_FILE

[obiwac]

Still not exactly the same behaviour as the Linux code, but I'm running this on Sway with wlroots' Vulkan renderer (which needs these for implicit sync interoperability) and it seems to work.

Maybe someone with a bit more LinuxKPI experience can tell me if there's a less hacky way to do this part:

/* We need to set these for linux_fget to work in sync_file_fdget. */
sync_file->linux_file->_file->f_ops = &linuxfileops;
sync_file->linux_file->_file->f_data = sync_file->linux_file;

This depends on #404.

[wulf7]

Here is some prior work: #357