engines/io_uring: set COOP_TASKRUN for ring setup

engines/io_uring: set COOP_TASKRUN for ring setup

If available, this will generally improve performance quite a bit.
Fio can easily use it, we just need to set the flag.
IORING_SETUP_COOP_TASKRUN tells the kernel that the application doesn't
need to get rescheduled to handle task_work, it's fine to defer this
to when we transition anyway.

For a QD=8 random read workload, this increases performance from
680K to 870K IOPS.

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