Fix bug that causes early termination of fio with verify_backlog

Fix bug that causes early termination of fio with verify_backlog

Running this job:

[global]
bs=4k
ioengine=sync
size=100m
direct=1
filename=xxx

[write]
verify=md5
verify_backlog=32
rw=write

causes fio to exit when reads + writes have exceeded 100m, where
it should only exit when writes have exceeded 100m. Fix this by
checking the appropriate counters based on what type of job it is.

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