selftests: forwarding: Add a test for pedit munge SIP and DIP

Add a test that checks that pedit adjusts source and destination
addresses of IPv4 and IPv6 packets.

Output example:

$ ./pedit_ip.sh
TEST: ping                                                          [ OK ]
TEST: ping6                                                         [ OK ]
TEST: dev swp2 ingress pedit ip src set 198.51.100.1                [ OK ]
TEST: dev swp3 egress pedit ip src set 198.51.100.1                 [ OK ]
TEST: dev swp2 ingress pedit ip dst set 198.51.100.1                [ OK ]
TEST: dev swp3 egress pedit ip dst set 198.51.100.1                 [ OK ]
TEST: dev swp2 ingress pedit ip6 src set 2001:db8:2::1              [ OK ]
TEST: dev swp3 egress pedit ip6 src set 2001:db8:2::1               [ OK ]
TEST: dev swp2 ingress pedit ip6 dst set 2001:db8:2::1              [ OK ]
TEST: dev swp3 egress pedit ip6 dst set 2001:db8:2::1               [ OK ]

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>