Linux: kmap() not page_address()

Linux: kmap() not page_address()

Using page_address() will return NULL if the page is in highmem. To
avoid this, we must kmap() the page we're getting the address of,
and kunmap() when we are done. If the page isn't in high memory, then
kmap() is equivalent to page_address().

Change-Id: I42abfadaf3101bf5ad41bd7e5f720ba2583c4ee5
Reviewed-on: http://gerrit.openafs.org/1705
Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit fdb9429eafda330bfdf1feefeb1a9ff61c7746b7)