net: context: prevent truncating outgoing datagrams

Datagrams should either be fully sent or not sent at all if networking
buffers or network interface MTU does not allow that. So far the behavior
was to truncate outgoing packets, even for datagram sockets.

When there is not enough available payload buffer to fit all requested
data, fail if that happens for datagram socket.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>