null_blk: add blocking mode

This adds a new module parameter to null_blk, blocking. If set, null_blk
will set the BLK_MQ_F_BLOCKING flag, indicating that it sometimes/always
needs to block in its ->queue_rq() function.  The intent is to help find
regressions in blocking drivers, since not many of them exist.

If null_blk is loaded with submit_queues > 1 and blocking=1, this
shows the regression recently fixed by bf4907c0.

Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Jens Axboe <axboe@fb.com>