sit: generate icmpv6 error when receiving icmpv4 error

Send icmpv6 error with type "destination unreachable" and code
"address unreachable" when receiving icmpv4 error and sufficient
data bytes are available
This patch enhances the compliance of sit tunnel with section 3.4 of
rfc 4213

Signed-off-by: Oussama Ghorbel <ghorbel@pivasoftware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>