fio: enable overlap checking with offload submission

fio: enable overlap checking with offload submission

Enable use of the serialize_overlap option with
io_submit_mode=offload. Threads with both of these options enabled will
check each other's in-flight IOs for overlap. If overlap is found,
submission of the new IO will be held back until the overlapping IO has
completed.

Threads must be used when fio is run in this mode because jobs need to
access each other's io_u's. io_u's are not allocated from the shared memory
area.

This fixes "fio serialize across jobs"
https://www.spinics.net/lists/fio/msg07133.html

Tested-by: Jeff Furlong <jeff.furlong@wdc.com>