s3/rpc_client: cleanup unmarshalling of variant types from row columns

s3/rpc_client: cleanup unmarshalling of variant types from row columns

Prior to this change fn 'extract_variant_addresses' actually returns offsets
to the variant stored not the addresses, additionally the param in the
signature of the method is named offset where the param in reality is a
base address.
This change makes fn 'extract_variant_addresses' actually return addresses
instead of offsets and also changes the name of the incoming param. The
resulting changes are propaged to callers which hopefully makes what the
code is actually doing a little clearer

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Noel Power <npower@samba.org>
Autobuild-Date(master): Tue Jan 30 17:22:37 UTC 2024 on atb-devel-224

(cherry picked from commit 9b2f2302ee4828ae54f5903a3bf649ffd255fb4a)