UPSTREAM: crypto: blake2b - merge _final implementation to callback

blake2b_final is called only once, merge it to the crypto API callback
and simplify. This avoids the temporary buffer and swaps the bytes of
internal buffer.

Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

(cherry picked from commit 086db43b)
Bug: 178411248
Change-Id: I7c5e2d9d931d463ada7573e66268f7b93a2d40a4
Signed-off-by: Eric Biggers <ebiggers@google.com>