selftests: forwarding: use proto icmp for {gretap, ip6gretap}_mac testing

For tc ip_proto filter, when we extract the flow via __skb_flow_dissect()
without flag FLOW_DISSECTOR_F_STOP_AT_ENCAP, we will continue extract to
the inner proto.

So for GRE + ICMP messages, we should not track GRE proto, but inner ICMP
proto.

For test mirror_gre.sh, it may make user confused if we capture ICMP
message on $h3(since the flow is GRE message). So I move the capture
dev to h3-gt{4,6}, and only capture ICMP message.

Before the fix:
]# ./mirror_gre.sh
TEST: ingress mirror to gretap (skip_hw)                            [ OK ]
TEST: egress mirror to gretap (skip_hw)                             [ OK ]
TEST: ingress mirror to ip6gretap (skip_hw)                         [ OK ]
TEST: egress mirror to ip6gretap (skip_hw)                          [ OK ]
TEST: ingress mirror to gretap: envelope MAC (skip_hw)              [FAIL]
 Expected to capture 10 packets, got 0.
TEST: egress mirror to gretap: envelope MAC (skip_hw)               [FAIL]
 Expected to capture 10 packets, got 0.
TEST: ingress mirror to ip6gretap: envelope MAC (skip_hw)           [FAIL]
 Expected to capture 10 packets, got 0.
TEST: egress mirror to ip6gretap: envelope MAC (skip_hw)            [FAIL]
 Expected to capture 10 packets, got 0.
TEST: two simultaneously configured mirrors (skip_hw)               [ OK ]
WARN: Could not test offloaded functionality

After fix:
]# ./mirror_gre.sh
TEST: ingress mirror to gretap (skip_hw)                            [ OK ]
TEST: egress mirror to gretap (skip_hw)                             [ OK ]
TEST: ingress mirror to ip6gretap (skip_hw)                         [ OK ]
TEST: egress mirror to ip6gretap (skip_hw)                          [ OK ]
TEST: ingress mirror to gretap: envelope MAC (skip_hw)              [ OK ]
TEST: egress mirror to gretap: envelope MAC (skip_hw)               [ OK ]
TEST: ingress mirror to ip6gretap: envelope MAC (skip_hw)           [ OK ]
TEST: egress mirror to ip6gretap: envelope MAC (skip_hw)            [ OK ]
TEST: two simultaneously configured mirrors (skip_hw)               [ OK ]
WARN: Could not test offloaded functionality

Fixes: ba8d3987 ("selftests: forwarding: Add test for mirror to gretap")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Petr Machata <pmachata@gmail.com>
Tested-by: Petr Machata <pmachata@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>