Add mutex lock for load confs in GetDefaultNetworks (#1073)

Thick server's chroot mutex is missing in GetDefaultNetworks,
that touch the pod filesystem. This change adds mutex lock there
and prevent race condition.

Fix #1072

pkg/types/conf.go

@@ -21,6 +21,7 @@ import (
 	"net"
 	"os"
 	"strings"
+	"sync"
 
 	"github.com/containernetworking/cni/libcni"
 	"github.com/containernetworking/cni/pkg/skel"
@@ -39,6 +40,9 @@ const (
 	defaultNonIsolatedNamespace   = "default"
 )
 
+// ChrootMutex provides lock to access host filesystem
+var ChrootMutex *sync.Mutex
+
 // LoadDelegateNetConfList reads DelegateNetConf from bytes
 func LoadDelegateNetConfList(bytes []byte, delegateConf *DelegateNetConf) error {
 	logging.Debugf("LoadDelegateNetConfList: %s, %v", string(bytes), delegateConf)