LU-13096 llite: serialize max_cached_mb write operation

LU-13096 llite: serialize max_cached_mb write operation

It is possible that two threads try to change max_cached_mb concurrently,
if they both try to reclaim LRU pages, it will be possible
that one thread finish, but another thread loop forever to reclaim
enough LRU pages.

Morever, this operations try to change two value, they should be
protected together to guarantee atomic, the whole operations could
be slow, this patch introduce another mutex lock to serialize operation.

Patch also fix to avoid reclaim too many LRU pages with large memory.

Change-Id: I1e88e37607f9e8db2ba67f1f7e5670f25c136465
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/37710
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>