btrfs: add helper for recording simple quota deltas

Rather than re-computing shared/exclusive ownership based on backrefs
and walking roots for implicit backrefs, simple quotas does an increment
when creating an extent and a decrement when deleting it. Add the API
for the extent item code to use to track those events.

Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>