ovl: fix error handling in ovl_verify_set_fh()

commit babf4770 upstream.

We hit a BUG on kfree of an ERR_PTR()...

Reported-by: syzbot+ff03fe05c717b82502d0@syzkaller.appspotmail.com
Fixes: 8b88a2e6 ("ovl: verify upper root dir matches lower root dir")
Cc: <stable@vger.kernel.org> # v4.13
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/overlayfs/namei.c

@@ -368,8 +368,10 @@ int ovl_verify_origin(struct dentry *dentry, struct vfsmount *mnt,
 
 	fh = ovl_encode_fh(origin, is_upper);
 	err = PTR_ERR(fh);
-	if (IS_ERR(fh))
+	if (IS_ERR(fh)) {
+		fh = NULL;
 		goto fail;
+	}
 
 	err = ovl_verify_origin_fh(dentry, fh);
 	if (set && err == -ENODATA)