eCryptfs: fix 32 bit corruption issue

Shifting page->index on 32 bit systems was overflowing, causing
data corruption of > 4GB files. Fix this by casting it first.

https://launchpad.net/bugs/1243636

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reported-by: Lars Duesing <lars.duesing@camelotsweb.de>
Cc: stable@vger.kernel.org # v3.11+
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>