blk-mq: refactor passthrough vs flush handling in blk_mq_insert_request

blk-mq: refactor passthrough vs flush handling in blk_mq_insert_request

While both passthrough and flush requests call directly into
blk_mq_request_bypass_insert, the parameters aren't the same.
Split the handling into two separate conditionals and turn the whole
function into an if/elif/elif/else flow instead of the gotos.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20230413064057.707578-11-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>