sit: fix an oops when IFLA_IPTUN_PROTO is not set

The use of this attribute has been added in 32b8a8e5 (sit: add IPv4 over
IPv4 support). It is optional, by default proto is IPPROTO_IPV6.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>