libata: don't flush dcache on slab pages

page_mapping() check this via VM_BUG_ON(PageSlab(page)) so we bug here
with the according debuging turned on.

Future TODO: replace this with a flush_dcache_page_for_pio() API

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Cc: stable@kernel.org