Fix. Prevents double close the same handle (#24)

lfs_unlock_dir can be called multiple times for the same object.
For example if lock:free is called manually.
Then lfs_unlock_dir will be called always
again, as soon as the LOCK_METATABLE is collected by GC.
This can lead to strange file errors later on, like closing
another file, which now has been assigned the same handle...
Merge pull request #28 from moteus/patch-1

Fix. Prevents double close the same handle (#24)