Kernel: Make sure outgoing ICMP packets have the correct checksum

The internet_checksum() function relies on the buffer - or at least the
checksum field - to be all zeroes.

Kernel/Net/NetworkTask.cpp

@@ -253,6 +253,7 @@ void handle_icmp(const EthernetFrameHeader& eth, const IPv4Packet& ipv4_packet,
  return;
  }
  adapter->fill_in_ipv4_header(*packet, adapter->ipv4_address(), eth.source(), ipv4_packet.source(), IPv4Protocol::ICMP, icmp_packet_size, 64);
+ memset(packet->buffer.data() + ipv4_payload_offset, 0, sizeof(ICMPEchoPacket));
  auto& response = *(ICMPEchoPacket*)(packet->buffer.data() + ipv4_payload_offset);
  response.header.set_type(ICMPType::EchoReply);
  response.header.set_code(0);