[NETFILTER]: Fix NAT PMTUD problems

ICMP errors are only SNATed when their source matches the source of the
connection they are related to, otherwise the source address is not
changed. This creates problems with ICMP frag. required messages
originating from a router behind the NAT, if private IPs are used the
packet has a good change of getting dropped on the path to its destination.

Always NAT ICMP errors similar to the original connection.

Based on report by Al Viro.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>