Don't block on STDIN open

This was found testing other runtime shims that are faster than runc(no
containerization).  This is a race that can cause the shim to block
forever.  It's not an issue for out/err because we open both sides of
the pipe, but for stdin, it expects the client to have it opened.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

runtime/v2/runc/service.go

@@ -98,8 +98,7 @@ type service struct {
 	ec        chan runcC.Exit
 	ep        *epoller
 
-	id string
-	// Filled by Create()
+	id     string
 	bundle string
 	cg     cgroups.Cgroup
 }