libct/utils: SearchLabels: optimize

Using strings.Split generates temporary strings for GC to collect.
Rewrite the function to not do that.

Also, add a second return value, so that the caller can distinguish
between an empty value found and no key found cases.

Fix the test accordingly.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/utils/utils.go

@@ -132,19 +132,16 @@ func WithProcfd(root, unsafePath string, fn func(procfd string) error) error {
 	return fn(procfd)
 }
 
-// SearchLabels searches a list of key-value pairs for the provided key and
-// returns the corresponding value. The pairs must be separated with '='.
-func SearchLabels(labels []string, query string) string {
-	for _, l := range labels {
-		parts := strings.SplitN(l, "=", 2)
-		if len(parts) < 2 {
-			continue
-		}
-		if parts[0] == query {
-			return parts[1]
+// SearchLabels searches through a list of key=value pairs for a given key,
+// returning its value, and the binary flag telling whether the key exist.
+func SearchLabels(labels []string, key string) (string, bool) {
+	key += "="
+	for _, s := range labels {
+		if strings.HasPrefix(s, key) {
+			return s[len(key):], true
 		}
 	}
-	return ""
+	return "", false
 }
 
 // Annotations returns the bundle path and user defined annotations from the