net/portmapper: mark fewer PMP probe failures as unexpected

There are lots of lines in the logs of the form:

portmapper: unexpected PMP probe response: {OpCode:128 ResultCode:3
SecondsSinceEpoch:NNN MappingValidSeconds:0 InternalPort:0
ExternalPort:0 PublicAddr:0.0.0.0}

ResultCode 3 here means a network failure, e.g. the NAT box itself has
not obtained a DHCP lease. This is not an indication that something
is wrong in the Tailscale client, so use different wording here
to reflect that. Keep logging, so that we can analyze and debug
the reasons that PMP probes fail.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

net/portmapper/portmapper.go

@@ -798,7 +798,12 @@ func (c *Client) Probe(ctx context.Context) (res ProbeResult, err error) {
 				c.logf("unexpected PCP probe response: %+v", pres)
 			}
 			if pres, ok := parsePMPResponse(buf[:n]); ok {
-				if pres.OpCode == pmpOpReply|pmpOpMapPublicAddr && pres.ResultCode == pmpCodeOK {
+				if pres.OpCode != pmpOpReply|pmpOpMapPublicAddr {
+					c.logf("unexpected PMP probe response opcode: %+v", pres)
+					continue
+				}
+				switch pres.ResultCode {
+				case pmpCodeOK:
 					c.logf("[v1] Got PMP response; IP: %v, epoch: %v", pres.PublicAddr, pres.SecondsSinceEpoch)
 					res.PMP = true
 					c.mu.Lock()
@@ -807,6 +812,10 @@ func (c *Client) Probe(ctx context.Context) (res ProbeResult, err error) {
 					c.pmpLastEpoch = pres.SecondsSinceEpoch
 					c.mu.Unlock()
 					continue
+				case pmpCodeNotAuthorized, pmpCodeNetworkFailure, pmpCodeOutOfResources:
+					// Normal failures.
+					c.logf("PMP probe failed due result code: %+v", pres)
+					continue
 				}
 				c.logf("unexpected PMP probe response: %+v", pres)
 			}