Change cast6-avx to use the new XTS code, for smaller stack usage and small
boost to performance.

tcrypt results, with Intel i5-2450M:
        enc     dec
16B     1.01x   1.01x
64B     1.01x   1.00x
256B    1.09x   1.02x
1024B   1.08x   1.06x
8192B   1.08x   1.07x
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>