Always request exclusive lock for the lower vnode in nullfs_vget().

The null_nodeget() requires exclusive lock on lowervp to be able to
insmntque() new vnode.

Reported by:	rea
Tested by:	pho
MFC after:	1 week

sys/fs/nullfs/null_vfsops.c

@@ -312,6 +312,12 @@ nullfs_vget(mp, ino, flags, vpp)
 	struct vnode **vpp;
 {
 	int error;
+
+	KASSERT((flags & LK_TYPE_MASK) != 0,
+	    ("nullfs_vget: no lock requested"));
+	flags &= ~LK_TYPE_MASK;
+	flags |= LK_EXCLUSIVE;
+
 	error = VFS_VGET(MOUNTTONULLMOUNT(mp)->nullm_vfs, ino, flags, vpp);
 	if (error)
 		return (error);