vppinfra: make _vec_len() read-only

Use of _vec_len() to set vector length breaks address sanitizer.
Users should use vec_set_len(), vec_inc_len(), vec_dec_len () instead.

Type: improvement
Change-Id: I441ae948771eb21c23a61f3ff9163bdad74a2cb8
Signed-off-by: Damjan Marion <damarion@cisco.com>
diff --git a/src/vppinfra/linux/mem.c b/src/vppinfra/linux/mem.c
index 6c3e036..9cf598d 100644
--- a/src/vppinfra/linux/mem.c
+++ b/src/vppinfra/linux/mem.c
@@ -261,7 +261,7 @@
 
   /* memfd_create maximum string size is 249 chars without trailing zero */
   if (vec_len (s) > 249)
-    _vec_len (s) = 249;
+    vec_set_len (s, 249);
   vec_add1 (s, 0);
 
   /* memfd_create introduced in kernel 3.17, we don't support older kernels */