afr: check for split-brain before proceeding with fops

Bail out of fops if split brain has been detected during lookup

Change-Id: Id387dbb1a25eec4a121dedceadc6069bdea24b5d
BUG: 1010834
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/5988
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>