Refactored lookup count decrementing.
Refactored lookup count decrementing.

BanzaiMan/gcsfuse,mbrukman/gcsfuse,BanzaiMan/gcsfuse,GoogleCloudPlatform/gcsfuse,jacobsa/timeutil,kahing/gcsfuse,jacobsa/ratelimit,GoogleCloudPlatform/gcsfuse,GoogleCloudPlatform/gcsfuse