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/devices/af_packet/node.c b/src/vnet/devices/af_packet/node.c
index d74e56f..8dbd855 100644
--- a/src/vnet/devices/af_packet/node.c
+++ b/src/vnet/devices/af_packet/node.c
@@ -191,7 +191,7 @@
   u32 frame_num = apif->rx_req->tp_frame_nr;
   u8 *block_start = apif->rx_ring + block * block_size;
   uword n_trace = vlib_get_trace_count (vm, node);
-  u32 thread_index = vlib_get_thread_index ();
+  u32 thread_index = vm->thread_index;
   u32 n_buffer_bytes = vlib_buffer_free_list_buffer_size (vm,
 							  VLIB_BUFFER_DEFAULT_FREE_LIST_INDEX);
   u32 min_bufs = apif->rx_req->tp_frame_size / n_buffer_bytes;