block: move bio_mergeable out of bio.h

bio_mergeable is only needed by I/O schedulers, so move it to
blk-mq-sched.h.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211012161804.991559-4-hch@lst.de


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