crypto: x86/aes-xts - make the fast path 64-bit specific

Remove 32-bit support from the fast path in xts_crypt().  Then optimize
it for 64-bit, and simplify the code, by switching to sg_virt() and
removing the now-unnecessary checks for crossing a page boundary.

The result is simpler code that is slightly smaller and faster in the
case that actually matters (64-bit).

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