ovl: fix dentry reference leak

In ovl_copy_up_locked(), newdentry is leaked if the function exits through
out_cleanup as this just to out after calling ovl_cleanup() - which doesn't
actually release the ref on newdentry.

The out_cleanup segment should instead exit through out2 as certainly
newdentry leaks - and possibly upper does also, though this isn't caught
given the catch of newdentry.

Without this fix, something like the following is seen:

	BUG: Dentry ffff880023e9eb20{i=f861,n=#ffff880023e82d90} still in use (1) [unmount of tmpfs tmpfs]
	BUG: Dentry ffff880023ece640{i=0,n=bigfile}  still in use (1) [unmount of tmpfs tmpfs]

when unmounting the upper layer after an error occurred in copyup.

An error can be induced by creating a big file in a lower layer with
something like:

	dd if=/dev/zero of=/lower/a/bigfile bs=65536 count=1 seek=$((0xf000))

to create a large file (4.1G).  Overlay an upper layer that is too small
(on tmpfs might do) and then induce a copy up by opening it writably.

Reported-by: Ulrich Obergfell <[email protected]>
Signed-off-by: David Howells <[email protected]>
Signed-off-by: Miklos Szeredi <[email protected]>
Cc: <[email protected]> # v3.18+

fs/overlayfs/copy_up.c

@@ -267,7 +267,7 @@ static int ovl_copy_up_locked(struct dentry *workdir, struct dentry *upperdir,
 
 out_cleanup:
 	ovl_cleanup(wdir, newdentry);
-	goto out;
+	goto out2;
 }
 
 /*