ci: enable checklocks workflow for specific packages

This turns the checklocks workflow into a real check, and adds
annotations to a few basic packages as a starting point.

Updates #12625

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I2b0185bae05a843b5257980fc6bde732b1bdd93f

net/stun/stuntest/stuntest.go

@@ -21,8 +21,10 @@ import (
 )
 
 type stunStats struct {
-	mu       sync.Mutex
+	mu sync.Mutex
+	// +checklocks:mu
 	readIPv4 int
+	// +checklocks:mu
 	readIPv6 int
 }