nullfs: remove the vhold/vdrop cycle around unlock

Both lower vnode and null data are safe while the lock is held,
at the same time neither is touched after unlock.

While here remove stale comment about interlock handling. It is no
longer legal to pass to unlock.

Reviewed by:		kib
Tested by:		pho (previous version)
Differential Revision:	https://reviews.freebsd.org/D38761

sys/fs/nullfs/null_vnops.c

@@ -840,11 +840,6 @@ null_lock(struct vop_lock1_args *ap)
 	return (error);
 }
 
-/*
- * We need to process our own vnode unlock and then clear the
- * interlock flag as it applies only to our vnode, not the
- * vnodes below us on the stack.
- */
 static int
 null_unlock(struct vop_unlock_args *ap)
 {
@@ -853,11 +848,20 @@ null_unlock(struct vop_unlock_args *ap)
 	struct vnode *lvp;
 	int error;
 
+	/*
+	 * Contrary to null_lock, we don't need to hold the vnode around
+	 * unlock.
+	 *
+	 * We hold the lock, which means we can't be racing against vgone.
+	 *
+	 * At the same time VOP_UNLOCK promises to not touch anything after
+	 * it finishes unlock, just like we don't.
+	 *
+	 * vop_stdunlock for a doomed vnode matches doomed locking in null_lock.
+	 */
 	nn = VTONULL(vp);
 	if (nn != NULL && (lvp = NULLVPTOLOWERVP(vp)) != NULL) {
-		vholdnz(lvp);
 		error = VOP_UNLOCK(lvp);
-		vdrop(lvp);
 	} else {
 		error = vop_stdunlock(ap);
 	}