HashIndex: in cleanup, interpret missing dir as completed merge

If we stop between unlinking the empty subdir and removing the root
merge marker, we get ENOENT on the get_info.  That's actually fine.

Backport: firefly
Fixes: 8332
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1835 from ceph/wip-8332

HashIndex: in cleanup, interpret missing dir as completed merge

Reviewed-by: Sage Weil <sage@inktank.com>