locks: Protected racy (read) access of ext_list

Change-Id: Ibf639695ebd99c11c6960c9be82c0cee71b50744
BUG: 905864
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/4458
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>