LU-12631 llite: report latency for filesystem ops

LU-12631 llite: report latency for filesystem ops

Add the elapsed time of VFS operations to the llite stats
counter, instead of just tracking the number of operations,
to allow tracking of operation round-trip latency.

Update sanity test_127[ab] to check that llite.*.stats and
osc.*.stats counter shows read/write stats in usec, and
fix code style nearby.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I40e188374f91c030d978a83157d8869e928cab07
Reviewed-on: https://review.whamcloud.com/36078
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>