FreeBSD: Ensure that z_pflags is initialized for new znodes

The field is subsequently accessed in zfs_mknode(), in
zfs_inherit_projid(). The Linux implementation of zfs_create_fs() has
this initialization already; there is no counterpart to
zfs_create_share_dir() that I can see.

Reported-by: KMSAN
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes #17486