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

ipn/store/mem/store_mem.go

@@ -20,7 +20,8 @@ func New(logger.Logf, string) (ipn.StateStore, error) {
 
 // Store is an ipn.StateStore that keeps state in memory only.
 type Store struct {
-	mu    sync.Mutex
+	mu sync.Mutex
+	// +checklocks:mu
 	cache map[ipn.StateKey][]byte
 }