Always update rusage before grabbing stat_mutex

Always update rusage before grabbing stat_mutex

zxh reports running into a rusage_sem vs stat_mutex deadlock:

"I am running into a deadlock with fio for the following lines:

thread_main

fio_mutex_down(stat_mutex) at backend.c:1529

helper_thread_main

fio_mutex_down(td->rusage_sem) at stat.c:1467

thread_main is waiting for stat_mutex, which is already locked by
helper_thread_main in function __show_running_run_stats() at
stat.c:1441. However, the helper_thread_main is waiting for
td->rusage_sem, which is supposed to be unlocked by
check_update_rusage() in do_io() at backend.c:1525 in thread_main.

The issue is not reproducible every time, and I was using a customized
ioengine derived from rbd.c.

Is there any chance that this issue is caused by the customized io
engine? Or is there a way to get around this?"

Fix this by always updating the rusage stats before grabbing
stat_mutex, so we avoid this ABBA deadlock.

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