vn_path_to_global_path_hardlink(): initialize len

before calling vn_fullpath_hardlink().  Otherwise we get random failures
when the len is automatically clipped.

Reported and tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

sys/kern/vfs_cache.c

@@ -3861,6 +3861,7 @@ vn_path_to_global_path_hardlink(struct thread *td, struct vnode *vp,
 	 * name.
 	 */
 	VOP_UNLOCK(vp);
+	len = pathlen;
 	error = vn_fullpath_hardlink(vp, dvp, leaf_name, leaf_length,
 	    &rpath, &fbuf, &len);