sit: fix 4in4 + IPsec scenario

Since commit 32b8a8e5

 "sit: add IPv4 over IPv4 support",
tunnel->parms.iph.protocol is 0 when both 4in4 and 6in4 are setup, but
xfrm_lookup() is called only when proto is != 0, thus we need to pass the real
value.

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