ovl: fix dentry leak in ovl_indexdir_cleanup()

index dentry was not released when breaking out of the loop
due to index verification error.

Fixes: 415543d5

 ("ovl: cleanup bad and stale index entries on mount")
Cc: <stable@vger.kernel.org> # v4.13
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>