Fix failure to verify in mixed read/write workload with backlog

Fix failure to verify in mixed read/write workload with backlog

If you run a workload like this:

fio --rw=randrw --bs=4k --direct=1 --ioengine=libaio --iodepth=32
--verify=meta --verify_backlog=1024 --verify_fatal=1 --name=ver-test
--filename=foo --size=1G --verify_pattern=0xaaa

Fio ends up never actually verifying the written blocks. This happens
because as we generate an entry to be verified, the backend checks
whether this is a read/write mixed workload. It then thinks that the
READ is just that, a normal READ, when in fact it could be coming
from our verify list.

Mark such a read as coming from our verify list, so that we know if
it's a "normal" read or one generated to verify previously written
data.

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