btrfs: make btrfs_dio_private::bytes u32

btrfs_dio_private::bytes is only assigned from bio::bi_iter::bi_size,
which is never larger than U32.
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/btrfs_inode.h

@@ -325,7 +325,8 @@ struct btrfs_dio_private {
 	struct inode *inode;
 	u64 logical_offset;
 	u64 disk_bytenr;
-	u64 bytes;
+	/* Used for bio::bi_size */
+	u32 bytes;
 
 	/*
 	 * References to this structure. There is one reference per in-flight