parisc: add mm API for DMA to vmalloc/vmap areas

We already have an API to flush a kernel page along an alias
address, so use it.  The TLB purge prevents the CPU from doing
speculative moveins on the flushed address, so we don't need to
implement and invalidate.

Acked-by: Kyle McMartin <kyle@mcmartin.ca>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>