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>