net: context: Fix the v4 mapped address handling in sendto

If we receive a IPv4 packet to v4 mapped address, the relevant
net_context is bound to IPv6. This causes issues if we try
to get the family from the context struct in sendto.
Fix this by checking if the destination address is IPv4 but
the socket is bound to IPv6 and v4 mapping is enabled.
If all these criterias are set, then set the family of the
packet separately and do not get it from net_context.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>