crypto: aesni - Add fast path for > 16 byte update

We can fast-path any < 16 byte read if the full message is > 16 bytes,
and shift over by the appropriate amount.  Usually we are
reading > 16 bytes, so this should be faster than the READ_PARTIAL
macro introduced in b20209c9

 for the average case.

Signed-off-by: Dave Watson <davejwatson@fb.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>