ipn/ipnlocal: check if suggested exit node is online

@nickkyl added an peer.Online check to suggestExitNodeUsingDERP, so it
should also check when running suggestExitNodeUsingTrafficSteering.

Updates tailscale/corp#29966

Signed-off-by: Simon Law <sfllaw@tailscale.com>

ipn/ipnlocal/local.go

@@ -7907,6 +7907,9 @@ func suggestExitNodeUsingTrafficSteering(nb *nodeBackend, prev tailcfg.StableNod
 		if !p.Valid() {
 			return false
 		}
+		if !p.Online().Get() {
+			return false
+		}
 		if allowed != nil && !allowed.Contains(p.StableID()) {
 			return false
 		}