cocci: Avoid name collisions in check_obj.cocci

I get a new warning with Coccinelle XXX:

> magic, previously declared as a metavariable, is used as an identifier

I don't remember seeing this warning before, but it looks like @@ blocks
may not behave as I thought. I didn't review the other semantic patches.

tools/coccinelle/check_obj.cocci

@@ -4,12 +4,12 @@
  */
 
 @@
-expression obj, magic;
+expression obj, magicval;
 @@
 
 if (obj != NULL) {
-- CHECK_OBJ_NOTNULL(obj, magic);
-+ CHECK_OBJ(obj, magic);
+- CHECK_OBJ_NOTNULL(obj, magicval);
++ CHECK_OBJ(obj, magicval);
 ...
 }