[netstack] Add a fallback name for network bridges

...in case one hasn't been configured. We can keep this code path the
same even if/when netcfg eventually ends up naming bridges, because
we'll want the affordance of seeing `brN` in ifconfig output if naming
fails in other ways.

Tested:
- sudo tunctl -u stijlist -t qemu-extra
- sudo ifconfig qemu-extra up
- fx run -kN -- -netdev type=tap,ifname=qemu-extra,script=no,downscript=no,id=net1 -device e1000,netdev=net1,mac=52:54:00:63:5e:7b
- fx shell "ifconfig bridge ethp0003 ethp0004"
- fx shell "ifconfig" | grep br4

Change-Id: Ief6f9db67720d90ca7294e0117073b5281b32bf3
diff --git a/go/src/netstack/netstack.go b/go/src/netstack/netstack.go
index ab66c96..c47c47e 100644
--- a/go/src/netstack/netstack.go
+++ b/go/src/netstack/netstack.go
@@ -454,7 +454,10 @@
 
 	return ns.addEndpoint(func(*ifState) (stack.LinkEndpoint, error) {
 		return bridge.New(links), nil
-	}, func(*ifState) error {
+	}, func(ifs *ifState) error {
+		if len(ifs.nic.Name) == 0 {
+			ifs.nic.Name = fmt.Sprintf("br%d", ifs.nic.ID)
+		}
 		return nil
 	})
 }