randstruct: Whitelist NIU struct page overloading

The NIU ethernet driver intentionally stores a page struct pointer on
top of the "mapping" field. Whitelist this case:

drivers/net/ethernet/sun/niu.c: In function ‘niu_rx_pkt_ignore’:
drivers/net/ethernet/sun/niu.c:3402:10: note: found mismatched ssa struct pointer types: ‘struct page’ and ‘struct address_space’

    *link = (struct page *) page->mapping;
    ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Kees Cook <keescook@chromium.org>

scripts/gcc-plugins/randomize_layout_plugin.c

@@ -46,6 +46,8 @@ struct whitelist_entry {
 };
 
 static const struct whitelist_entry whitelist[] = {
+	/* NIU overloads mapping with page struct */
+	{ "drivers/net/ethernet/sun/niu.c", "page", "address_space" },
 	/* unix_skb_parms via UNIXCB() buffer */
 	{ "net/unix/af_unix.c", "unix_skb_parms", "char" },
 	/* big_key payload.data struct splashing */