kernel: add missing kfree, use proper return code

* This is half picked from 559be41482
* We haven't sync latest upstream changes yet

Signed-off-by: Faris <rissu.ntk@gmail.com>

kernel/supercalls.c

@@ -348,7 +348,7 @@ static int do_set_feature(void __user *arg)
 static int do_get_wrapper_fd(void __user *arg)
 {
 	if (!ksu_file_sid) {
-		return -1;
+		return -EINVAL;
 	}
 
 	const char *anon_name = "[ksu_fdwrapper]";