fio: add function to check for serialize_overlap with offload submission

fio: add function to check for serialize_overlap with offload submission

In multiple places fio needs to check whether it is carrying out overlap
checking in offload submission mode. Make this check a function to
improve code readability.

Reviewed-by: Sebastien Boisvert <sboisvert@gydle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>