block: factor out a blk_try_enter_queue helper

block: factor out a blk_try_enter_queue helper

Factor out the code to try to get q_usage_counter without blocking into
a separate helper.  Both to improve code readability and to prepare for
splitting bio_queue_enter from blk_queue_enter.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20210929071241.934472-3-hch@lst.de
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>