sit tunnels: propagate IPv6 transport class to IPv4 Type of Service

sit tunnels (IPv6 tunnel over IPv4) do not implement the "tos inherit"
case to copy the IPv6 transport class byte from the inner packet to
the IPv4 type of service byte in the outer packet. By contrast, ipip
tunnels and GRE tunnels do.

This patch, adapted from the similar code in net/ipv4/ipip.c and
net/ipv4/ip_gre.c, implements that.

This patch applies to 3.0.1, and has been tested on that version.

Signed-off-by: Lionel Elie Mamane <lionel@mamane.lu>
Signed-off-by: David S. Miller <davem@davemloft.net>