niu: Silence randstruct warnings

niu: Silence randstruct warnings

Clang randstruct gets upset when it sees struct addresspace (which is
randomized) being assigned to a struct page (which is not randomized):

drivers/net/ethernet/sun/niu.c:3385:12: error: casting from randomized structure pointer type 'struct address_space *' to 'struct page *'
                        *link = (struct page *) page->mapping;
                                ^

It looks like niu.c is looking for an in-line place to chain its allocated
pages together and is overloading the "mapping" member, as it is unused.
This is very non-standard, and is expected to be cleaned up in the
future[1], but there is no "correct" way to handle it today.

No meaningful machine code changes result after this change, and source
readability is improved.

Drop the randstruct exception now that there is no "confusing" cross-type
assignment.

[1] https://lore.kernel.org/lkml/YnqgjVoMDu5v9PNG@casper.infradead.org/

Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Du Cheng <ducheng2@gmail.com>
Cc: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: William Kucharski <william.kucharski@oracle.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: netdev@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-hardening@vger.kernel.org
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/lkml/20220511151647.7290adbe@kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>