cluster/afr: perform list-xattr during lookup

Detect and heal mismatching user extended attributes during lookup.
'Forward' port of http://review.gluster.org/#/c/7444/

Change-Id: Id03c9746f083ffd3014711d0b3a2e5a71a45eed4
BUG: 1134691
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/8558
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Tested-by: Pranith Kumar Karampuri <pkarampu@redhat.com>