[XTENSA] Fix modules for non-exec processor configurations

We need to use vmalloc_exec for module loading. Also remove
the definitions MODULE_START and MODULE_END, which wasn't
used, and increase the VMALLOC memory range accordingly.

Signed-off-by: Chris Zankel <chris@zankel.net>

arch/xtensa/kernel/module.c

@@ -28,7 +28,7 @@ void *module_alloc(unsigned long size)
 {
 	if (size == 0)
 		return NULL;
-	return vmalloc(size);
+	return vmalloc_exec(size);
 }
 
 void module_free(struct module *mod, void *module_region)