module: fix clang CFI with MODULE_UNLOAD=n

When CONFIG_MODULE_UNLOAD is disabled, the module->exit member
is not defined, causing a build failure:

kernel/module.c:4493:8: error: no member named 'exit' in 'struct module'
                mod->exit = *exit;

add an #ifdef block around this.

Fixes: cf68fffb ("add support for Clang CFI")
Acked-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jessica Yu <jeyu@kernel.org>

kernel/module.c

@@ -4489,8 +4489,10 @@ static void cfi_init(struct module *mod)
 	/* Fix init/exit functions to point to the CFI jump table */
 	if (init)
 		mod->init = *init;
+#ifdef CONFIG_MODULE_UNLOAD
 	if (exit)
 		mod->exit = *exit;
+#endif
 
 	cfi_module_add(mod, module_addr_min);
 #endif