filestore: fix collection_move behavior

This appears to fix the problem.
Sort of.. the caller is already replaying the op (this whole dance is done by _collection_move, which does the various fsync's to avoid replay.  The only pieces we need to handle are having two simultanous links to the same inode that can both be looked up via long file names.  And having two those names potentially both be in the same short_name chain (if the sha1 hash collides).