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>
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index b72711a..9b1f4ef 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2694,8 +2694,8 @@ int open_ctree(struct super_block *sb,
 	btrfs_init_balance(fs_info);
 	btrfs_init_async_reclaim_work(&fs_info->async_reclaim_work);
 
-	sb->s_blocksize = 4096;
-	sb->s_blocksize_bits = blksize_bits(4096);
+	sb->s_blocksize = BTRFS_BDEV_BLOCKSIZE;
+	sb->s_blocksize_bits = blksize_bits(BTRFS_BDEV_BLOCKSIZE);
 
 	btrfs_init_btree_inode(fs_info);
 
@@ -3316,7 +3316,7 @@ int btrfs_read_dev_one_super(struct block_device *bdev, int copy_num,
 	if (bytenr + BTRFS_SUPER_INFO_SIZE >= i_size_read(bdev->bd_inode))
 		return -EINVAL;
 
-	bh = __bread(bdev, bytenr / 4096, BTRFS_SUPER_INFO_SIZE);
+	bh = __bread(bdev, bytenr / BTRFS_BDEV_BLOCKSIZE, BTRFS_SUPER_INFO_SIZE);
 	/*
 	 * If we fail to read from the underlying devices, as of now
 	 * the best option we have is to mark it EIO.
@@ -3409,7 +3409,7 @@ static int write_dev_supers(struct btrfs_device *device,
 		btrfs_csum_final(crc, sb->csum);
 
 		/* One reference for us, and we leave it for the caller */
-		bh = __getblk(device->bdev, bytenr / 4096,
+		bh = __getblk(device->bdev, bytenr / BTRFS_BDEV_BLOCKSIZE,
 			      BTRFS_SUPER_INFO_SIZE);
 		if (!bh) {
 			btrfs_err(device->fs_info,
@@ -3468,7 +3468,8 @@ static int wait_dev_supers(struct btrfs_device *device, int max_mirrors)
 		    device->commit_total_bytes)
 			break;
 
-		bh = __find_get_block(device->bdev, bytenr / 4096,
+		bh = __find_get_block(device->bdev,
+				      bytenr / BTRFS_BDEV_BLOCKSIZE,
 				      BTRFS_SUPER_INFO_SIZE);
 		if (!bh) {
 			errors++;