btrfs: fix uninitialized variable warning in btrfs_cleanup_ordered_extents

We can conditionally pass in a locked page, and then we'll use that page
range to skip marking errors as that will happen in another layer.
However this causes the compiler to complain because it doesn't
understand we only use these values when we have the page.  Make the
compiler stop complaining by setting these values to 0.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>