crypto: x86/aes-xts - simplify loop in xts_crypt_slowpath()

Since the total length processed by the loop in xts_crypt_slowpath() is
a multiple of AES_BLOCK_SIZE, just round the length down to
AES_BLOCK_SIZE even on the last step.  This doesn't change behavior, as
the last step will process a multiple of AES_BLOCK_SIZE regardless.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>