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>