btrfs: use named constant for bdev blocksize

Superblock is read and written using buffer heads, we need to set the
bdev blocksize. The magic constant has been hardcoded in several places,
so replace it with a named constant.

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.h

@@ -25,6 +25,14 @@
 #define BTRFS_SUPER_MIRROR_MAX	 3
 #define BTRFS_SUPER_MIRROR_SHIFT 12
 
+/*
+ * Fixed blocksize for all devices, applies to specific ways of reading
+ * metadata like superblock. Must meet the set_blocksize requirements.
+ *
+ * Do not change.
+ */
+#define BTRFS_BDEV_BLOCKSIZE	(4096)
+
 enum btrfs_wq_endio_type {
 	BTRFS_WQ_ENDIO_DATA = 0,
 	BTRFS_WQ_ENDIO_METADATA = 1,