ovl: relax WARN_ON() on rename to self

In ovl_rename(), if new upper is hardlinked to old upper underneath
overlayfs before upper dirs are locked, user will get an ESTALE error
and a WARN_ON will be printed.

Changes to underlying layers while overlayfs is mounted may result in
unexpected behavior, but it shouldn't crash the kernel and it shouldn't
trigger WARN_ON() either, so relax this WARN_ON().

Reported-by: syzbot+bb1836a212e69f8e201a@syzkaller.appspotmail.com
Fixes: 804032fa ("ovl: don't check rename to self")
Cc: <stable@vger.kernel.org> # v4.9+
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/dir.c

@@ -1170,7 +1170,7 @@ static int ovl_rename(struct inode *olddir, struct dentry *old,
 	if (newdentry == trap)
 		goto out_dput;
 
-	if (WARN_ON(olddentry->d_inode == newdentry->d_inode))
+	if (olddentry->d_inode == newdentry->d_inode)
 		goto out_dput;
 
 	err = 0;