MAINTAINERS: add bio.h to the block section

bio.h is part of the block layer, so list it in the MAINTAINERS file
as such.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220127064221.1314477-1-hch@lst.de


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