dns/dnsmessage: update SVCB packing to prohibit name compression

This commit updates the dns/dnsmessage package to correctly handle SVCB record
packing by prohibiting name compression for the Target field.

It turns out the compression of names in RDATA is prohibited (see
https://datatracker.ietf.org/doc/html/rfc3597#section-4), except for well-known record
types (those defined in https://datatracker.ietf.org/doc/html/rfc1035).

The SVCB RFC actually explicitly calls that out
(https://datatracker.ietf.org/doc/html/rfc9460#section-2.2), so it was an oversight
in my part in https://go.dev/cl/710736.

Updates golang/go#43790

Change-Id: I287b034c9e01b49264e07e23293bacd830ab1832
GitHub-Last-Rev: 1d16664
GitHub-Pull-Request: #242
Reviewed-on: https://go-review.googlesource.com/c/net/+/713900
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Vinicius Fortuna <fortuna@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Sean Liao <sean@liao.dev>

Author: fortuna

dns/dnsmessage/svcb.go

@@ -160,10 +160,14 @@ func (k SVCParamKey) GoString() string {
 	return printUint16(uint16(k))
 }
 
-func (r *SVCBResource) pack(msg []byte, compression map[string]uint16, compressionOff int) ([]byte, error) {
+func (r *SVCBResource) pack(msg []byte, _ map[string]uint16, _ int) ([]byte, error) {
 	oldMsg := msg
 	msg = packUint16(msg, r.Priority)
-	msg, err := r.Target.pack(msg, compression, compressionOff)
+	// https://datatracker.ietf.org/doc/html/rfc3597#section-4 prohibits name
+	// compression for RR types that are not "well-known".
+	// https://datatracker.ietf.org/doc/html/rfc9460#section-2.2 explicitly states that
+	// compression of the Target is prohibited, following RFC 3597.
+	msg, err := r.Target.pack(msg, nil, 0)
 	if err != nil {
 		return oldMsg, &nestedError{"SVCBResource.Target", err}
 	}