[dhcpv6] Do not set sin6_scope_id on the unspecified client socket address

Setting sin6_scope_id to a non-zero value will cause the check against
the "empty socket address" in udp_demux() to fail, and incoming DHCPv6
responses on interfaces other than net0 will be rejected with a
spurious "No UDP connection listening on port 546" error.

The transmitting network device is specified via the destination
address, not the source address.  Fix by simply not setting
sin6_scope_id on the client socket address.

Reported-by: Anton D. Kachalov <mouse@yandex-team.ru>
Tested-by: Anton D. Kachalov <mouse@yandex-team.ru>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/udp/dhcpv6.c

 	/* Construct client and server addresses */
 	memset ( &addresses, 0, sizeof ( addresses ) );
 	addresses.client.sin6.sin6_family = AF_INET6;
-	addresses.client.sin6.sin6_scope_id = netdev->index;
 	addresses.client.sin6.sin6_port = htons ( DHCPV6_CLIENT_PORT );
 	addresses.server.sin6.sin6_family = AF_INET6;
 	ipv6_all_dhcp_relay_and_servers ( &addresses.server.sin6.sin6_addr );