net/netcheck: don't send flood of PCP unmap requests to router

Updates #810

(cherry picked from commit 05e5233e07)

net/netcheck/netcheck.go

@@ -715,6 +715,7 @@ func (rs *reportState) probePortMapServices() {
 	uc.WriteTo(pcpPacket(myIP, tempPort, false), port5351)
 
 	res := make([]byte, 1500)
+	sentPCPDelete := false
 	for {
 		n, addr, err := uc.ReadFrom(res)
 		if err != nil {
@@ -732,6 +733,8 @@ func (rs *reportState) probePortMapServices() {
 			if n == 60 && res[0] == 0x02 { // right length and version 2
 				rs.setOptBool(&rs.report.PCP, true)
 
+				if !sentPCPDelete {
+					sentPCPDelete = true
 					// And now delete the mapping.
 					// (PCP is the only protocol of the three that requires
 					// we cause a side effect to detect whether it's present,
@@ -741,6 +744,7 @@ func (rs *reportState) probePortMapServices() {
 			}
 		}
 	}
+}
 
 var pmpPacket = []byte{0, 0} // version 0, opcode 0 = "Public address request"