LU-1585 lnet: Fix an incorrect timestamp calculation in lst.c

LU-1585 lnet: Fix an incorrect timestamp calculation in lst.c

The operation in routine lst_timeval_diff() (in lst.c) has
a bug. It uses tv_sec where it should be using tv_usec.

Signed-off-by: Doug Oucharek <doug@whamcloud.com>
Change-Id: I898dacd3d4a2c84594148d0514fda731d24a49bf
Reviewed-on: http://review.whamcloud.com/3474
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Liang Zhen <liang@whamcloud.com>
Reviewed-by: Isaac Huang <he.huang@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>