FROMGIT: crypto: poly1305 - add Poly1305 core API

Expose a low-level Poly1305 API which implements the
ε-almost-∆-universal (εA∆U) hash function underlying the Poly1305 MAC
and supports block-aligned inputs only.

This is needed for Adiantum hashing, which builds an εA∆U hash function
from NH and a polynomial evaluation in GF(2^{130}-5); this polynomial
evaluation is identical to the one the Poly1305 MAC does.  However, the
crypto_shash Poly1305 API isn't very appropriate for this because its
calling convention assumes it is used as a MAC, with a 32-byte "one-time
key" provided for every digest.

But by design, in Adiantum hashing the performance of the polynomial
evaluation isn't nearly as critical as NH.  So it suffices to just have
some C helper functions.  Thus, this patch adds such functions.

Acked-by: Martin Willi <martin@strongswan.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

(cherry picked from commit 1b6fd3d5d18bbc1b1abf3b0cbc4b95a9a63d407b
 https://git.kernel.org/pub/scm/linux/kernel/git/herbert/cryptodev-2.6.git

 master)
Bug: 112008522
Test: As series, see Ic61c13b53facfd2173065be715a7ee5f3af8760b
Change-Id: I5c7da7832b84dfe29c300e117a158740d3e39069
Signed-off-by: Eric Biggers <ebiggers@google.com>