FreeBSD: Don't remove SA xattr if not SA znode

We attempt to remove an existing SA xattr when setting a dir xattr, but
this only makes sense if the znode has been upgraded to the SA format.
Otherwise, we will hit an assert in zfs_sa_get_xattr.

Make sure this is an SA znode before attempting to remove the SA xattr.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #12514