block: move io_context creation into where it's needed

The only user of the io_context for IO is BFQ, yet we put the checking
and logic of it into the normal IO path.

Put the creation into blk_mq_sched_assign_ioc(), and have BFQ use that
helper.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>