crypto: x86/aegis128 - take advantage of block-aligned len

Update a caller of aegis128_aesni_ad() to round down the length to a
block boundary.  After that, aegis128_aesni_ad(), aegis128_aesni_enc(),
and aegis128_aesni_dec() are only passed whole blocks.  Update the
assembly code to take advantage of that, which eliminates some unneeded
instructions.  For aegis128_aesni_enc() and aegis128_aesni_dec(), the
length is also always nonzero, so stop checking for zero length.

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