crypto: hctr2 - Convert to use POLYVAL library

crypto: hctr2 - Convert to use POLYVAL library

The "hash function" in hctr2 is fixed at POLYVAL; it can never vary.
Just use the POLYVAL library, which is much easier to use than the
crypto_shash API.  It's faster, uses fixed-size structs, and never fails
(all the functions return void).

Note that this eliminates the only known user of the polyval support in
crypto_shash.  A later commit will remove support for polyval from
crypto_shash, given that the library API is sufficient.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20251109234726.638437-7-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>