lib/crypto: blake2s: Replace manual unrolling with unrolled_full

lib/crypto: blake2s: Replace manual unrolling with unrolled_full

As we're doing in the BLAKE2b code, use unrolled_full to make the
compiler handle the loop unrolling.  This simplifies the code slightly.
The generated object code is nearly the same with both gcc and clang.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20251205051155.25274-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>