zfs_file: implement zfs_file_deallocate for FreeBSD 14

FreeBSD 14 gained a VOP_DEALLOCATE VFS operation and a fspacectl
syscall to use it. At minimum, these zero the given region, and if the
underlying filesystem supports it, can make the region sparse. We can
use this to get TRIM-like behaviour for file vdevs.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16496