Add the missing support for ciphertext stealing in the implementation
of AES-XTS, which is part of the XTS specification but was omitted up
until now due to lack of a need for it.

The asm helpers are updated so they can deal with any input size, as
long as the last full block and the final partial block are presented
at the same time. The glue code is updated so that the common case of
operating on a sector or page is mostly as before. When CTS is needed,
the walk is split up into two pieces, unless the entire input is covered
by a single step.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>