param: initialize store function to NULL if not available.

I rebased Kees' 'param: do not set store func without write perm'
on top of my 'params: cleanup sysfs allocation'.  However, my patch
uses krealloc which doesn't zero memory, leaving .store unset.
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

kernel/params.c

@@ -648,6 +648,8 @@ static __modinit int add_sysfs_param(struct module_kobject *mk,
 	/* Do not allow runtime DAC changes to make param writable. */
 	if ((kp->perm & (S_IWUSR | S_IWGRP | S_IWOTH)) != 0)
 		mk->mp->attrs[mk->mp->num].mattr.store = param_attr_store;
+	else
+		mk->mp->attrs[mk->mp->num].mattr.store = NULL;
 	mk->mp->attrs[mk->mp->num].mattr.attr.name = (char *)name;
 	mk->mp->attrs[mk->mp->num].mattr.attr.mode = kp->perm;
 	mk->mp->num++;