crypto: arm64/aes-neonbs - implement ciphertext stealing for XTS

Update the AES-XTS implementation based on NEON instructions so that it
can deal with inputs whose size is not a multiple of the cipher block
size. This is part of the original XTS specification, but was never
implemented before in the Linux kernel.

Since the bit slicing driver is only faster if it can operate on at
least 7 blocks of input at the same time, let's reuse the alternate
path we are adding for CTS to process any data tail whose size is
not a multiple of 128 bytes.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>