btrfs: rename tree_ref and data_ref owning_root

commit 113479d5

 ("btrfs: rename root fields in delayed refs structs")
changed these from ref_root to owning_root. However, there are many
circumstances where that name is not really accurate and the root on the
ref struct _is_ the referring root. In general, these are not the owning
root, though it does happen in some ref merging cases involving
overwrites during snapshots and similar.

Simple quotas cares quite a bit about tracking the original owner of an
extent through delayed refs, so rename these back to free up the name
for the real owning root (which will live on the generic btrfs_ref and
the head ref)

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>