blk-mq: remove blk-mq-tag.h

blk-mq-tag.h is always included by blk-mq.h, and causes recursive
inclusion hell with further changes.  Just merge it into blk-mq.h
instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20230413064057.707578-3-hch@lst.de


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