crypto: adiantum - Use scatter_walk API instead of sg_miter

crypto: adiantum - Use scatter_walk API instead of sg_miter

Make adiantum_hash_message() use the scatter_walk API instead of
sg_miter.  scatter_walk is a bit simpler and also more efficient.  For
example, unlike sg_miter, scatter_walk doesn't require that the number
of scatterlist entries be calculated up-front.

Link: https://lore.kernel.org/r/20251211011846.8179-8-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>