sit: add IPv4 over IPv4 support

This patch adds the support of IPv4 over Ipv4 for the module sit. The gain of
this feature is to be able to have 4in4 and 6in4 over the same interface
instead of having one interface for 6in4 and another for 4in4 even if
encapsulation addresses are the same.

To avoid conflicting with ipip module, sit IPv4 over IPv4 protocol is
registered with a smaller priority.

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