BACKPORT: Do not hold the peer map lock during the peerDBNetworkWalk

Upstream reference: moby/libnetwork#1638
Upstream reference: moby#31016
BZ https://bugzilla.redhat.com/show_bug.cgi?id=1446184

Signed-off-by: Antonio Murdaca <runcom@redhat.com>

vendor/src/github.com/docker/libnetwork/drivers/overlay/peerdb.go

@@ -80,25 +80,30 @@ func (d *driver) peerDbWalk(f func(string, *peerKey, *peerEntry) bool) error {
 func (d *driver) peerDbNetworkWalk(nid string, f func(*peerKey, *peerEntry) bool) error {
 	d.peerDb.Lock()
 	pMap, ok := d.peerDb.mp[nid]
+	d.peerDb.Unlock()
+
 	if !ok {
-		d.peerDb.Unlock()
 		return nil
 	}
-	d.peerDb.Unlock()
+
+	mp := map[string]peerEntry{}
 
 	pMap.Lock()
 	for pKeyStr, pEntry := range pMap.mp {
+		mp[pKeyStr] = pEntry
+	}
+	pMap.Unlock()
+
+	for pKeyStr, pEntry := range mp {
 		var pKey peerKey
 		if _, err := fmt.Sscan(pKeyStr, &pKey); err != nil {
 			log.Warnf("Peer key scan on network %s failed: %v", nid, err)
 		}
 
 		if f(&pKey, &pEntry) {
-			pMap.Unlock()
 			return nil
 		}
 	}
-	pMap.Unlock()
 
 	return nil
 }