time: fix overflow in timeval_add_msec()

time: fix overflow in timeval_add_msec()

There is an issue in FIO when using the detailed bandwidth and iops
logging with averaging over a period of time > 1 second.  It seems that
usecs overflows which later causes negative time diff values resulting
in skewed toward 0 results.  I have attached a potential fix that should
prevent usecs from going beyond 1000000.

[Modified by Jens to put the adj_sec in the branch.]

Signed-off-by: Jens Axboe <axboe@fb.com>