icmp: fix ExtendedEchoRequest extension object

RFC8335 [1] Sec 2: "When applied to the ICMP
Extended Echo Request message, the ICMP Extension Structure MUST
contain exactly one instance of the Interface Identification Object."

The InterfaceIdent by name object seems to be a copy/paste issue.

[1]: https://www.rfc-editor.org/rfc/rfc8335.txt

Change-Id: I1ef5f38382ebede0cb3ee792892673f6c2b39672
GitHub-Last-Rev: 73f963c5dc1c83bef08fdcfa89a3608e7345f3c4
GitHub-Pull-Request: golang/net#24
Reviewed-on: https://go-review.googlesource.com/c/146638
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

icmp/multipart_test.go

@@ -232,11 +232,6 @@ func TestMarshalAndParseMultipartMessage(t *testing.T) {
 							Type:  2,
 							Index: 911,
 						},
-						&icmp.InterfaceIdent{
-							Class: 3,
-							Type:  1,
-							Name:  "en101",
-						},
 					},
 				},
 			},