FileStore: CollectionIndex Lock is hold for the Path lifetime
The RWLock is not of much benefit here since the lock is already
been sharded in the Index level. The lock is acquired in the Path
constructor based on a parameter and released in the Path destructor.
So, if Index->lookup() called within the scope of the previous Path
object, the lock should not be asked to held. This is caller's
responsibility. In lfn_open() is called with a valid Index object,
the Path lock will not be held.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>