crypto: scatterwalk - Avoid flush_dcache_page on slab pages

It's illegal to call flush_dcache_page on slab pages on a number
of architectures.  So this patch avoids doing so if PageSlab is
true.

In future we can move the flush_dcache_page call to those page
cache users that actually need it.
Reported-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/scatterwalk.c

@@ -54,6 +54,7 @@ static void scatterwalk_pagedone(struct scatter_walk *walk, int out,
 		struct page *page;
 
 		page = sg_page(walk->sg) + ((walk->offset - 1) >> PAGE_SHIFT);
+		if (!PageSlab(page))
 			flush_dcache_page(page);
 	}