helper_thread: Increase timer accuracy

helper_thread: Increase timer accuracy

The helper_thread uses select() to wait for the next event. select() rounds
up its timeout argument to 1 / HZ. Increase timer accuracy by using a
high-res timer if possible. This patch has been tested by running the
following fio command and by verifying that the next_log and
msec_to_next_event were always above 100 ms:

./fio --debug=helperthread --bandwidth-log --write_bw_log=log --log_avg_msec=1000 --ioengine=null --name=null --size=1G --rw=randwrite --loops=999999 --runtime=60

Signed-off-by: Bart Van Assche <bvanassche@acm.org>