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 openzfs#12514

module/os/freebsd/zfs/zfs_vnops_os.c

@@ -5695,7 +5695,7 @@ zfs_setextattr(struct vop_setextattr_args *ap)
 	}
 	if (error) {
 		error = zfs_setextattr_dir(ap, attrname);
-		if (error == 0)
+		if (error == 0 && zp->z_is_sa)
 			/*
 			 * Successfully put into dir, we need to clear the one
 			 * in SA if present.