bcache: struct cache_sb is only for in-memory super block now

We have struct cache_sb_disk for on-disk super block already, it is
unnecessary to keep the in-memory super block format exactly mapping
to the on-disk struct layout.

This patch adds code comments to notice that struct cache_sb is not
exactly mapping to cache_sb_disk, and removes the useless member csum
and pad[5].

Although struct cache_sb does not belong to uapi, but there are still
some on-disk format related macros reference it and it is unncessary to
get rid of such dependency now. So struct cache_sb will continue to stay
in include/uapi/linux/bache.h for now.

Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>