Do not use bypass for vop_vptocnp() from nullfs, call standard

implementation instead. The bypass does not assume that returned vnode
is only held.

Reported by:	Paul B. Mahol <onemda gmail com>, pluknet <pluknet gmail com>
Reviewed by:	jhb
Tested by:	pho, pluknet <pluknet gmail com>

sys/fs/nullfs/null_vnops.c

@@ -742,5 +742,6 @@ struct vop_vector null_vnodeops = {
 	.vop_setattr =		null_setattr,
 	.vop_strategy =		VOP_EOPNOTSUPP,
 	.vop_unlock =		null_unlock,
+	.vop_vptocnp =		vop_stdvptocnp,
 	.vop_vptofh =		null_vptofh,
 };