nullfs: stop special-casing directories in null_vptocnp

The previous code was forcing an expensive walk in vop_stdvptocnp,
which was causing performance issues on highly contended zfs.

No objections:	kib
MFC after:	2 weeks

sys/fs/nullfs/null_vnops.c

@@ -870,9 +870,6 @@ null_vptocnp(struct vop_vptocnp_args *ap)
 	struct ucred *cred = ap->a_cred;
 	int error, locked;
 
-	if (vp->v_type == VDIR)
-		return (vop_stdvptocnp(ap));
-
 	locked = VOP_ISLOCKED(vp);
 	lvp = NULLVPTOLOWERVP(vp);
 	vhold(lvp);