block: split out operations on block special files

Add a new block/fops.c for all the file and address_space operations
that provide the block special file support.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210907141303.1371844-2-hch@lst.de


[axboe: correct trailing whitespace while at it]
Signed-off-by: Jens Axboe <axboe@kernel.dk>