eCryptfs: Decrypt pages in-place

When reading in a page, eCryptfs would allocate a helper page, fill it
with encrypted data from the lower filesytem, and then decrypt the data
from the encrypted page and store the result in the eCryptfs page cache
page.

The crypto API supports in-place crypto operations which means that the
allocation of the helper page is unnecessary when decrypting. This patch
gets rid of the unneeded page allocation by reading encrypted data from
the lower filesystem directly into the page cache page. The page cache
page is then decrypted in-place.

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>