Avoid re-use of page index as tweak for AES-XTS when multiple parts of
same page are encrypted. This will happen on multiple (partial) calls of
fscrypt_encrypt_page on same page.
page->index is only valid for writeback pages.
Signed-off-by: NDavid Gstir <david@sigma-star.at>
Signed-off-by: NRichard Weinberger <richard@nod.at>
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>