LU-176 obdclass: disable mb_cache by default

LU-176 obdclass: disable mb_cache by default

We are supposed to disable mb_cache by default (bug 22771),
but we never did because it's not in default mount option.
This patch will add "no_mbcache" to default mount option.

Change-Id: I13c9db8e98ad305d26d887d8bd069eff92c20763
Signed-off-by: Liang Zhen <liang@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/373
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>