btrfs: backref: rename and move finish_upper_links()

This the the 2nd major part of generic backref cache. Move it to
backref.c so we can reuse it.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
3 files changed