The only possible error return from null_nodeget() is due to insmntque1

failure (the getnewvnode cannot return an error). In this case, the
null_insmntque_dtr() already unlocked the reclaimed vnode, so VOP_UNLOCK()
in the nullfs_mount() after null_nodeget() failure is wrong.

Tested by:	pho
MFC after:	1 week

sys/fs/nullfs/null_vfsops.c

@@ -157,7 +157,6 @@ nullfs_mount(struct mount *mp)
 	 * Make sure the node alias worked
 	 */
 	if (error) {
-		VOP_UNLOCK(vp, 0);
 		vrele(lowerrootvp);
 		free(xmp, M_NULLFSMNT);	/* XXX */
 		return (error);