Revert "icmp: fix ExtendedEchoRequest extension object"

This reverts commit e1173011.

Reason for revert: The reverted test case is one of typical wrong wire
format test cases. The exposed API intentionally doesn't provide any
extenion object validation because the API is also used to construct
wire format compliance test tools. The API is extesion object-agnostic
and should be able to transmit and receive payload containing extension
objects in wrong wire format. Please preserve such test cases for now.

If you want to drop such test case, please add 1) extension object
validation, 2) a control knob for skipping validation, then drop all of
them.

Change-Id: I5c488c95523488e511e7a91e29a2f24f08448415
Reviewed-on: https://go-review.googlesource.com/c/146877Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

icmp/multipart_test.go

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