dht: fix dht_discover_cbk doing a wrong layout set.

with the sequence of operations are like below, we have issues
with current code (MP == mountpoint):
T0,MP1# mkdir /abcd (Succeeds on hash_subvol)
T1,MP2# mkdir /abcd (Gets EEXIST as dir exists in hash_subvol)
T2,MP2# mkdir /.gfid/<abcd's gfid>/xyz (lookup happens on abcd's
        gfid, calls dht_discover)
T3,MP1# (Completes mkdir(), goes to dir_selfheal to set the layouts).
T4,MP2# (dht_discover_cbk gets success for lookup as the entry
        existed, as layout is not yet written, it says normalize
        done, found holes).
T5,MP2# (as layout anomaly is not considered an issue in this patch,
        dht_layout_set happens on inode, with all xlators pointing
        to 0s)
T6,MP1# (completes mkdir call, inode has proper layouts)
T7,MP2# mkdir /.gfid/<abcd's gfid>/xyz fails with ENOENT
        (with log saying no subvol found for hash value of xyz.

Porting Amar's fix from down-stream beta branch.

Change-Id: Ibdc37ee614c96158a1330af19cad81a39bee2651
BUG: 982913
Original-author: Amar Tumballi <amarts@redhat.com>
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/5302
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>