features/quota: unref and set priv->rpc_clnt to NULL once disabled.

Change-Id: Ic1a6bc36b26cf31f00644bd674b24daa11745e64
BUG: 1103636
Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-on: http://review.gluster.org/7876
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>