Cache only addresses of whole pages, not of each buffer chunk.  Besides,
page addresses can be obtained by page_address() instead of kmap() since
they were allocated in lowmem.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>