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/udp/udp.c b/src/vnet/udp/udp.c
index 888a6cb..9618936 100644
--- a/src/vnet/udp/udp.c
+++ b/src/vnet/udp/udp.c
@@ -265,7 +265,7 @@
 {
   udp_main_t *um = vnet_get_udp_main ();
   vlib_main_t *vm = vlib_get_main ();
-  u32 thread_index = vlib_get_thread_index ();
+  u32 thread_index = vm->thread_index;
   udp_connection_t *uc;
   ip46_address_t lcl_addr;
   u32 node_index;