cluster/dht: Set layout when inode is present

Problem:
Lookups in discovery fail with ENOENT so local->inode
is never set. dht_layout_set logs the callstack when
the function is called in that state.

Fix:
Don't set layout when lookups fail in discovery.

Change-Id: I5d588314c89e3575fcf7796d57847e35fd20f89a
BUG: 965434
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/5055
Reviewed-by: Shishir Gowda <sgowda@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>