net: coap: Adjust offset value after packet parsing

It is important that offset is set in the same manner, regardless of the
origin packet - it should indicate the final packet length in both
cases, when the packet is generated on the Zephyr side with CoAP APIs,
and when it's parsed from the UDP datagram. This allows for functions
like `coap_packet_get_payload()` to work correcty in both cases.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>