crypto: xts - Convert to skcipher

This patch converts xts over to the skcipher interface.  It also
optimises the implementation to be based on ECB instead of the
underlying cipher.  For compatibility the existing naming scheme
of xts(aes) is maintained as opposed to the more obvious one of
xts(ecb(aes)).

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>