[PARISC] fix vmap flush/invalidate

On parisc, we never implemented invalidate_kernel_vmap_range() because
it was unnecessary for the xfs use case.  However, we do need to
implement an invalidate for the opposite use case (which occurred in a
recent NFS change) where the user wants to read through the vmap range
and write via the kernel address.  There's an additional complexity to
this in that if the page has no userspace mappings, it might have dirty
cache lines in the kernel (indicated by the PG_dcache_dirty bit).  In
order to get full coherency, we need to flush these pages through the
kernel mapping before invalidating the vmap range.

Signed-off-by: James Bottomley <James.Bottomley@suse.de>