znode: expose zfs_get_zplprop to libzpool

There's no particular reason this function should be kernel-only, and I
want to use it (indirectly) from zdb. I've moved it to zfs_znode.c
because libzpool does not compile in zfs_vfsops.c, and this at least
matches the header its imported from.

Sponsored-By: Klara, Inc.
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: WHR <msl0000023508@gmail.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #14642