os/: Add CollectionIndex::prep_delete

os/: Add CollectionIndex::prep_delete

If an unlink is interupted between removing the file
and updating the subdir attribute, the attribute will
overestimate the number of files in the directory.  This
is by design, at worst we will merge the collection later
than intended, but closing the gap would require a second
subdir xattr update.  However, this can in extreme cases
result in a collection with subdirectories but no objects.
FileStore::_destry_collection would therefore see an
erroneous -ENOTEMPTY.

prep_delete allows the CollectionIndex implementation to
clean up state prior to removal.

Signed-off-by: Samuel Just <sam.just@inktank.com>