net: dns: Add per socket user data for the dispatcher

The socket services API has a limitation where the user data is shared
between file descriptors described in the same service.

This can cause problem in DNS dispatcher where each listened socket
needs to have their own dispatcher struct set as user data so that we
can map between dispatcher context and socket. Solve this by always
have a dispatcher table as user data, and then have the actual mapping
done via the dispatcher table when receiving data to the dispatcher socket.

Fixes #78146

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>
(cherry picked from commit caf9fd34)