[TCP] IPV6: fix softnet build breakage

net/ipv6/tcp_ipv6.c: In function 'tcp_v6_rcv':
net/ipv6/tcp_ipv6.c:1736: error: implicit declaration of function
'get_softnet_dma'
net/ipv6/tcp_ipv6.c:1736: warning: assignment makes pointer from integer
without a cast

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>