Refactored FileInode.Flush in preparation for sharing with Sync.
Refactored FileInode.Flush in preparation for sharing with Sync.

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