LU-1872 ptlrpc: cleanup jobstats in thread context

LU-1872 ptlrpc: cleanup jobstats in thread context

Since cfs_hash function isn't safe in interrupt context, we should
move the jobstats cleanup work into each stats logging function.

If there are any activities, cleanup will be triggerred on logging
time, and the hash table will be shrinked automatically; If there
isn't any activity, the hash table will stop growing, and it can
be cleared manually.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: I1c60e123f67c0ad3b959bbb330282cd07c63a28c
Reviewed-on: http://review.whamcloud.com/3945
Tested-by: Hudson
Reviewed-by: Liang Zhen <liang@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>