[IPV6]: Avoid calling ip6_xmit() with NULL sk

The ip6_xmit() function now assumes that its sk argument is non-NULL,
which isn't currently true when TCPv6 code is sending RST or ACK
packets. This fixes that code to use a socket of its own for sending
such packets, as TCPv4 does. (Thanks Andi for the pointer).

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>