avoid using thread local storage for thread index

It is cheaper to get thread index from vlib_main_t if available...

Change-Id: I4582e160d06d9d7fccdc54271912f0635da79b50
Signed-off-by: Damjan Marion <damarion@cisco.com>
diff --git a/src/vnet/l2tp/l2tp.c b/src/vnet/l2tp/l2tp.c
index 3dedc44..a59c789 100644
--- a/src/vnet/l2tp/l2tp.c
+++ b/src/vnet/l2tp/l2tp.c
@@ -157,7 +157,7 @@
   u32 session_index;
   u32 counter_index;
   u32 nincr = 0;
-  u32 thread_index = vlib_get_thread_index ();
+  u32 thread_index = vm->thread_index;
 
   /* *INDENT-OFF* */
   pool_foreach (session, lm->sessions,