Description of problem:
=======================
commithash mechanism which lookup optimize uses is not assigned to a newly created volume. 
With cluster.lookup-optimize enabled default in code for 3.4.0, when a new volume is created dht.commithash xattr should be assigned to the volume.

getfattr -d -e hex -m . /bricks/brick*/commit-*/
getfattr: Removing leading '/' from absolute path names
# file: bricks/brick0/commit-b0/
security.selinux=0x73797374656d5f753a6f626a6563745f723a676c7573746572645f627269636b5f743a733000
trusted.gfid=0x00000000000000000000000000000001
trusted.glusterfs.dht=0x0000000100000000000000007ffffffe
trusted.glusterfs.volume-id=0x85e3f07e1e2244b3abafafdccbd446f9

Version-Release number of selected component (if applicable):
3.12.2-7.el7rhgs.x86_64

How reproducible:
always

Steps to Reproduce:
===================
1) On 3.12.2-7.el7rhgs.x86_64 (this build has lookup-optimize enabled by default in code), create a x3 volume.
2) FUSE mount it on a client and do a lookup.
3) Check for commithash xattr on the backend bricks.

Actual results:
===============
dht.commithash xattr is not assigned to a newly created volume.

Expected results:
=================
when a new volume is created with lookup-optimize enabled, dht.commithash xattr should be assigned to the volume.