btrfs: mark the len field in struct btrfs_ordered_sum as unsigned

[ Upstream commit 6e4b2479

 ]

len can't ever be negative, so mark it as an u32 instead of int.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: 9e65bfca

 ("btrfs: fix qgroup_free_reserved_data int overflow")
Signed-off-by: Sasha Levin <sashal@kernel.org>