crypto: scatterwalk - Hide PageSlab call to optimise away flush_dcache_page

On architectures where flush_dcache_page is not needed, we will
end up generating all the code up to the PageSlab call.  This is
because PageSlab operates on a volatile pointer and thus cannot
be optimised away.

This patch works around this by checking whether flush_dcache_page
is needed before we call PageSlab which then allows PageSlab to be
compiled awy.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>