btrfs: make btrfs_super_block size match BTRFS_SUPER_INFO_SIZE

It's a common practice to avoid use sizeof(struct btrfs_super_block)
(3531), but to use BTRFS_SUPER_INFO_SIZE (4096).

The problem is that, sizeof(struct btrfs_super_block) doesn't match
BTRFS_SUPER_INFO_SIZE from the very beginning.

Furthermore, for all call sites except selftests, we always allocate
BTRFS_SUPER_INFO_SIZE space for super block, there isn't any real reason
to use the smaller value, and it doesn't really save any space.

So let's get rid of such confusing behavior, and unify those two values.

This modification also adds a new static_assert() to verify the size,
and moves the BTRFS_SUPER_INFO_* macros to the definition of
btrfs_super_block for the static_assert().
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -218,6 +218,9 @@ struct btrfs_root_backup {
 	u8 unused_8[10];
 } __attribute__ ((__packed__));
 
+#define BTRFS_SUPER_INFO_OFFSET			SZ_64K
+#define BTRFS_SUPER_INFO_SIZE			4096
+
 /*
  * the super block basically lists the main trees of the FS
  * it currently lacks any block count etc etc
@@ -270,7 +273,11 @@ struct btrfs_super_block {
 	__le64 reserved[28];
 	u8 sys_chunk_array[BTRFS_SYSTEM_CHUNK_ARRAY_SIZE];
 	struct btrfs_root_backup super_roots[BTRFS_NUM_BACKUP_ROOTS];
+
+	/* Padded to 4096 bytes */
+	u8 padding[565];
 } __attribute__ ((__packed__));
+static_assert(sizeof(struct btrfs_super_block) == BTRFS_SUPER_INFO_SIZE);
 
 /*
  * Compat flags that we support.  If any incompat flags are set other than the

fs/btrfs/disk-io.h

@@ -6,9 +6,6 @@
 #ifndef BTRFS_DISK_IO_H
 #define BTRFS_DISK_IO_H
 
-#define BTRFS_SUPER_INFO_OFFSET SZ_64K
-#define BTRFS_SUPER_INFO_SIZE 4096
-
 #define BTRFS_SUPER_MIRROR_MAX	 3
 #define BTRFS_SUPER_MIRROR_SHIFT 12