crypto: x86/xts - implement support for ciphertext stealing

Align the x86 code with the generic XTS template, which now supports
ciphertext stealing as described by the IEEE XTS-AES spec P1619.

Tested-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>