zfs_znode: lift common code to a single shared file

For now, userspace has no znode implementation. Some of the property and
path handling code is used there though and is the same on all
platforms, so we only need a single copy of it.

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #16492