[POWERPC] Fixes for the SLB shadow buffer code

On a machine with hardware 64kB pages and a kernel configured for a
64kB base page size, we need to change the vmalloc segment from 64kB
pages to 4kB pages if some driver creates a non-cacheable mapping in
the vmalloc area.  However, we never updated with SLB shadow buffer.
This fixes it.  Thanks to paulus for finding this.

Also added some write barriers to ensure the shadow buffer contents
are always consistent.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>