ftrace: Drop a VLA in module_exists()

Avoid a VLA by using a real constant expression instead of a variable.
The compiler should be able to optimize the original code and avoid using
an actual VLA. Anyway this change is useful because it will avoid a false
positive with -Wvla, it might also help the compiler generating better
code.

Link: http://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
Link: http://lkml.kernel.org/r/1522399988-8815-1-git-send-email-s.mesoraca16@gmail.comSigned-off-by: Salvatore Mesoraca <s.mesoraca16@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -3902,14 +3902,13 @@ static bool module_exists(const char *module)
 {
 	/* All modules have the symbol __this_module */
 	const char this_mod[] = "__this_module";
-	const int modname_size = MAX_PARAM_PREFIX_LEN + sizeof(this_mod) + 1;
-	char modname[modname_size + 1];
+	char modname[MAX_PARAM_PREFIX_LEN + sizeof(this_mod) + 2];
 	unsigned long val;
 	int n;
 
-	n = snprintf(modname, modname_size + 1, "%s:%s", module, this_mod);
+	n = snprintf(modname, sizeof(modname), "%s:%s", module, this_mod);
 
-	if (n > modname_size)
+	if (n > sizeof(modname) - 1)
 		return false;
 
 	val = module_kallsyms_lookup_name(modname);