Description of problem:
With the feature http://review.gluster.org/#/c/7702/ the on disk layout would now carry 0 in the first 4 bytes of the xattr trusted.glusterfs.dht. In previous revisions this was set to 1 and the DHT code does a strict check on the same.

As a result lookups would fail to read and throw layout errors, when older clients are used against a 3.7 volume.

Version-Release number of selected component (if applicable):
3.7

How reproducible:
Always

Steps to Reproduce:
1. Access a gluster volume using client bits from 3.7
2. Create a directory
3. Access the created directory from a client using 3.6 client bits

Actual results:
Step 3 reports layout inconsistencies

Expected results:
Step 3 should not report layout inconsistencies

Additional info:
Fixing this has a few parts to it as follows,
- Do not set layout xattr with the commit-hash unless lookup-unhashed is set to auto
- Do not allow older clients to connect if the above option is set
- Warn the administrator that setting lookup-unhashed to auto would cause older clients to not function well with the volume