ovl: cleanup temp index entries

A previous failed attempt to create or whiteout a directory index may
leave index entries named '#%x' in the index dir. Cleanup those temp
entries on mount instead of failing the mount.

In the future, we may drop 'work' dir and use 'index' dir instead.
This change is enough for cleaning up copy up leftovers 'from the future',
but it is not enough for cleaning up rmdir leftovers 'from the future'
(i.e. temp dir containing whiteouts).

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>