Introduce READ_PARTIAL_BLOCK macro, and use it in the two existing
partial block cases: AAD and the end of ENC_DEC.   In particular,
the ENC_DEC case should be faster, since we read by 8/4 bytes if
possible.

This macro will also be used to read partial blocks between
enc_update and dec_update calls.
Signed-off-by: Dave Watson <davejwatson@fb.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>