libnetlink: fix rtnl_send_check()

We currently cannot flush more than ~64 addresses on a device, with strange status.

# ip -stats addr flush dev eth3
Failed to send flush request: Success
Flush terminated

Problem is in rtnl_send_check(), where a return -1; is misplaced.

After patch, we can flush 64000 addresses without any problem.

# ip -stats addr flush dev eth3

*** Round 1, deleting 32338 addresses ***

*** Round 2, deleting 16168 addresses ***

*** Round 3, deleting 8082 addresses ***

*** Round 4, deleting 4058 addresses ***

*** Round 5, deleting 2028 addresses ***

*** Round 6, deleting 918 addresses ***

*** Round 7, deleting 278 addresses ***

*** Round 8, deleting 130 addresses ***
*** Flush is complete after 8 rounds ***

---

index b68e2fd..100dd40 100644
@@ -137,8 +137,8 @@ int rtnl_send_check(struct rtnl_handle *rth, const char *buf, int len)
 				fprintf(stderr, "ERROR truncated\n");
 			else 
 				errno = -err->error;
+			return -1;
 		}
-		return -1;
 	}
 
 	return 0;