fio: enable cross-thread overlap checking with processes

fio: enable cross-thread overlap checking with processes

Overlap checking with io_submit_mode=offload requires relevant jobs to
access each other's io_u's and io_u_all members. This patch modifies the
fio_memalign and io_u_queue helpers to include an indicator signifying
whether operations should use the shared memory pool. When fio is
carrying out cross-job overlap checking in offload submission mode,
these variables will be allocated from shared memory so that processes
can be used and threads will no longer be required.

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