UPSTREAM: crypto: blake2b - open code set last block helper

The helper is trival and called once, inlining makes things simpler.
There's a comment to tie it back to the idea behind the code.

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

(cherry picked from commit a2e4bdce0f6e69c6cf9e460c4fe158bcc0db351f)
Bug: 178411248
Change-Id: Iba1276dfcdb29240206f66bf98902d0e587a590f
Signed-off-by: Eric Biggers <ebiggers@google.com>