fix append to uninitialized buffer in FlatIndex::created

The long_name variable is not initialized. When the append_oname
function is called, it will strlen(long_name) and get a result
that depends on the stack content. The long_name is truncated to a
zero length string to prevent this unexpected behavior.

There is no sure way to trigger the problem by writing a unit
test. Unit tests are added for all public methods of the FlatIndex
class. Most of the time the tests fail if the long_name variable is
not properly initialized.

 * uint32_t collection_version()
 * coll_t coll() const
 * void set_ref(std::tr1::shared_ptr<CollectionIndex> ref)
 * int cleanup()
 * int init()
 * int created(const hobject_t &hoid, const char *path)
 * int unlink(const hobject_t &hoid)
 * int lookup(const hobject_t &hoid, IndexedPath *path, int *exist)
 * int collection_list(vector<hobject_t> *ls)
 * int collection_list_partial(const hobject_t &start, int min_count, int max_count, snapid_t seq, vector<hobject_t> *ls, hobject_t *next)

There are a number of border cases that cannot be tested, such as the
logic of the lfn_get static function. Since FlatIndex code is designed
to transition from older namespace conventions.

The tests rely on xattr(2) and their availability is checked before
running them.

http://tracker.ceph.com/issues/4542 refs #4542

Signed-off-by: Loic Dachary <loic@dachary.org>