crypto: scatterwalk - Hide PageSlab call to optimise away flush_dcache_page

crypto: scatterwalk - Hide PageSlab call to optimise away flush_dcache_page

Orabug: 25243093

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>
(cherry picked from commit 160544075f2a4028209721723a51f16add7b08b9)
Signed-off-by: Ethan Zhao <ethan.zhao@oracle.com>