vlib: introduce user flags in vlib_frame_t

Those flags have local significance and they can be used for
sending hints to the next node.

Change-Id: Ic2596ee81c64cd16f96344365370e8fcdc465354
Signed-off-by: Damjan Marion <damarion@cisco.com>
diff --git a/src/vnet/ip/ip4_reassembly.c b/src/vnet/ip/ip4_reassembly.c
index a88ee2f..3466f72 100644
--- a/src/vnet/ip/ip4_reassembly.c
+++ b/src/vnet/ip/ip4_reassembly.c
@@ -1286,7 +1286,7 @@
 	      n_left_to_next -= 1;
 	      IP4_REASS_DEBUG_BUFFER (bi, enqueue_drop_timeout_walk);
 	    }
-	  f->flags |= (trace_frame * VLIB_FRAME_TRACE);
+	  f->frame_flags |= (trace_frame * VLIB_FRAME_TRACE);
 	  vlib_put_frame_to_node (vm, rm->ip4_drop_idx, f);
 	}