MAINTAINERS: add fs/block_dev.c to the block section

fs/block_dev.c is a pretty integral part of the block layer, so make
sure it is mentioned in MAINTAINERS.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>