MINOR: pool: move pool declarations to read_mostly

All pool heads are accessed via a pointer and should not be shared with
highly written variables. Move them to the read_mostly section.
diff --git a/addons/ot/src/scope.c b/addons/ot/src/scope.c
index 462c865..ace8a22 100644
--- a/addons/ot/src/scope.c
+++ b/addons/ot/src/scope.c
@@ -20,9 +20,9 @@
 #include "include.h"
 
 
-static struct pool_head *pool_head_ot_scope_span = NULL;
-static struct pool_head *pool_head_ot_scope_context = NULL;
-static struct pool_head *pool_head_ot_runtime_context = NULL;
+static struct pool_head *pool_head_ot_scope_span __read_mostly = NULL;
+static struct pool_head *pool_head_ot_scope_context __read_mostly = NULL;
+static struct pool_head *pool_head_ot_runtime_context __read_mostly = NULL;
 
 #ifdef USE_POOL_OT_SCOPE_SPAN
 REGISTER_POOL(&pool_head_ot_scope_span, "ot_scope_span", sizeof(struct flt_ot_scope_span));