block: async_bio_lock does not need to be bh-safe

async_bio_lock is only taken from bio submission and workqueue context,
both are never in bottom halves.

Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>