Fix bandwidth logging for mixed read/write workloads.

Fix bandwidth logging for mixed read/write workloads.

fio was maintaining separate read/write stats, but only one timer for
bw_avg_time. Whichever IO direction happened to cross the timer would
get its interval logged; the other data direction would not. Now both
ddir are logged each time we cross bw_avg_time.

Where intervals don't contain any activity in a given ddir, no log
entry is made.

Signed-off-by: Jens Axboe <axboe@kernel.dk>