zfs_file: rename zfs_file_fallocate to zfs_file_deallocate

We only use it on a specific way: to punch a hole in (make sparse) a
region of a file, in order to implement TRIM-like behaviour.

So, call the op "deallocate", and move the Linux-style mode flags down
into the Linux implementation, since they're an implementation detail.

FreeBSD gets a no-op stub (for the moment).

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