nfs, afr: Fail lookup only on split-brain

Change-Id: Icee9772f1f1bf5336eb82a4dc13e198424cd4a65
BUG: 921996
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/4699
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>