log: fix for crash with rate IO and logging

log: fix for crash with rate IO and logging

If we run with an IO rate, and it happens to be low enough that
we never need to wait for the queue full condition, we will never
regrow the log and hence crash in stat.c on this line:

assert(iolog->pending->nr_samples < iolog->pending->max_samples);

Fix this by shuffling the code around a bit, so io_u_quiesce()
regrows the log, if needed, after having reaped completions.

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