Changed the DirInode.LookUpChild signature.
Changed the DirInode.LookUpChild signature.

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