bfq: use bfq_bic_lookup in bfq_limit_depth

No need to create a new I/O context if there is none present yet in
->limit_depth.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211126115817.2087431-5-hch@lst.de


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