CHROMIUM: verity: don't call page_address on unmapped highmem pages

CHROMIUM: verity: don't call page_address on unmapped highmem pages

This fixes a bug that is preventing ARM systems from booting with verity.

We were calling page_address on what could potentially have been an
unmapped highmem page. Fix is to pass the struct page to the crypto code
and let crypto handle the kmap/kunmap.

BUG=chromium-os:14089,chrome-os-partner:3287
TEST=Ran dm-verity.git unit tests. Ran platform_DMVerityCorruption on H/W.
Verified that ARM now boots.

Change-Id: Id2074052aab9eec8fa0aead39aded2b40cd19bad
Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
R=wad@chromium.org,olofj@chromium.org,taysom@chromium.org

Review URL: http://codereview.chromium.org/6835032