Don't try to call cdevsw d_close() method when devfs_close() is called

because of insmntque1() failure.

Found with:	stress2
Suggested and reviewed by:	kib

sys/fs/devfs/devfs_vnops.c

@@ -458,6 +458,13 @@ devfs_close(struct vop_close_args *ap)
 	struct cdevsw *dsw;
 	int vp_locked, error;
 
+	/*
+	 * XXX: Don't call d_close() if we were called because of
+	 * XXX: insmntque1() failure.
+	 */
+	if (vp->v_data == NULL)
+		return (0);
+
 	/*
 	 * Hack: a tty device that is a controlling terminal
 	 * has a reference from the session structure.