ovl: fix memory leak on unlink of indexed file

commit 63e132528032ce937126aba591a7b37ec593a6bb upstream.

The memory leak was detected by kmemleak when running xfstests
overlay/051,053

Fixes: caf70cb2 ("ovl: cleanup orphan index entries")
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>