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>

include/uapi/linux/bcache.h

@@ -215,8 +215,13 @@ struct cache_sb_disk {
 	__le64			d[SB_JOURNAL_BUCKETS];	/* journal buckets */
 };
 
+/*
+ * This is for in-memory bcache super block.
+ * NOTE: cache_sb is NOT exactly mapping to cache_sb_disk, the member
+ *       size, ordering and even whole struct size may be different
+ *       from cache_sb_disk.
+ */
 struct cache_sb {
-	__u64			csum;
 	__u64			offset;	/* sector where this sb was written */
 	__u64			version;
 
@@ -236,8 +241,6 @@ struct cache_sb {
 	__u64			feature_incompat;
 	__u64			feature_ro_compat;
 
-	__u64			pad[5];
-
 	union {
 	struct {
 		/* Cache devices */
@@ -245,7 +248,6 @@ struct cache_sb {
 
 		__u16		block_size;	/* sectors */
 		__u16		bucket_size;	/* sectors */
-
 		__u16		nr_in_set;
 		__u16		nr_this_dev;
 	};