block: don't defer flushes on blk-mq + scheduling

For blk-mq with scheduling, we can potentially end up with ALL
driver tags assigned and sitting on the flush queues. If we
defer because of an inlfight data request, then we can deadlock
if that data request doesn't already have a tag assigned.

This fixes a deadlock with running the xfs/297 xfstest, where
thousands of syncs can cause the drive queue to stall.

Signed-off-by: Jens Axboe <axboe@fb.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>