btrfs: make remove_extent_backref pass the root

With extent tree v2 we'll have a different extent root based on where
the bytenr is located, so adjust the remove_extent_backref() helper and
it's helpers to pass the extent_root around.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>