net: context: Verify if CONFIG_NET_SOCKETS_PACKET_DGRAM is enabled

Verify if CONFIG_NET_SOCKETS_PACKET_DGRAM is enabled when creating a
datagram packet socket. Otherwise, it's possible to create
non-functional AF_PACKET/SOCK_DGRAM socket w/o an error.

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