wgengine/bench: hold lock in TrafficGen.GotPacket while calling first packet callback

Without any synchronization here, the "first packet" callback can
be delayed indefinitely, while other work continues.
Since the callback starts the benchmark timer, this could skew results.
Worse, if the benchmark manages to complete before the benchmark timer begins,
it'll cause a data race with the benchmark shutdown performed by package testing.
That is what is reported in #1881.

This is a bit unfortunate, in that it means that users of TrafficGen have
to be careful to keep this callback speedy and lightweight and to avoid deadlocks.

Fixes #1881

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

wgengine/bench/trafficgen.go

@@ -180,6 +180,7 @@ func (t *TrafficGen) Generate(b []byte, ofs int) int {
 // GotPacket processes a packet that came back on the receive side.
 func (t *TrafficGen) GotPacket(b []byte, ofs int) {
 	t.mu.Lock()
+	defer t.mu.Unlock()
 
 	s := &t.cur
 	seq := int64(binary.BigEndian.Uint64(
@@ -203,9 +204,6 @@ func (t *TrafficGen) GotPacket(b []byte, ofs int) {
 
 	f := t.onFirstPacket
 	t.onFirstPacket = nil
-
-	t.mu.Unlock()
-
 	if f != nil {
 		f()
 	}