crypto: x86/aes-xts - change len parameter to int

The AES-XTS assembly code currently treats the length as signed, since
this saves a few instructions in the loop compared to treating it as
unsigned.  Therefore update the type to make this clear.  (It is not
actually passed any values larger than PAGE_SIZE.)

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