backend: only terminate multiple loops if we did no IO

Commit 095196b1e76b introduced a stop point if do_io() returned
nothing, but that only counts writes and trims. Change that to
count all types of IO.

Fixes: 095196b1e76b ("backend: terminate loop if we didn't do IO")
Signed-off-by: Jens Axboe <axboe@fb.com>