LU-1370 obdecho: few fixes about md_echo clients

LU-1370 obdecho: few fixes about md_echo clients

1. Remove LU_NOREF flag from echo client ctxt, because it
conflicts with cl ctxt(which has no NOREF flag). So if
echo_client and normal lustre client share the same context
cache, this flag will cause echo module refcount not being
dropped.

2. Ma_lmm_size in ma will be changed during mdd_xxx,
so we can not use that to free the ma_lmm.

3. Remove lu_device from linkage on lu_site during cleanup
 phase.

Signed-off-by: Di Wang <di.wang@whamcloud.com>
Change-Id: I08fec689ef9c8b64284f7400ab0fc4dc362ebf87
Reviewed-on: http://review.whamcloud.com/2784
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>