After #944 and #963 which was in the end a false-positive
I finally found (by luck) a real change from SLES12-SP1 to SP2
that actually requires an appropriate enhancement
in Relax-and-Recover:

Since SLES12-SP2 btrfs quota via "snapper setup-quota" is used
which means "snapper setup-quota" must be called
during "rear recover" if that was used in the original system.

I.e. during "rear mkbackup" I need to autodetect whether or not
btrfs quota via "snapper setup-quota" is used and if yes store that
info somehow in disklayout.conf (I assume filesystem quota info
belongs to disklayout.conf) so that "rear recover" could call
"snapper setup-quota" when needed.

Regarding how btrfs quota is used for snapper see
http://snapper.io/2016/05/18/space-aware-cleanup.html

For the reasoning behind why btrfs quota is used for snapper see
https://lists.opensuse.org/opensuse-factory/2016-09/msg00074.html
https://lists.opensuse.org/opensuse-factory/2016-09/msg00121.html
https://lists.opensuse.org/opensuse-factory/2016-09/msg00124.html

For details probably best is to read the whole (very interesting)
"btrfs quotas enabled by default" mail thread in Sep 2016
on the [email protected] mailing list.