BACKPORT: crypto: blake2b - update file comment

The file comment for blake2b_generic.c makes it sound like it's the
reference implementation of BLAKE2b with only minor changes.  But it's
actually been changed a lot.  Update the comment to make this clearer.

Reviewed-by: David Sterba <dsterba@suse.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

(cherry picked from commit 0cdc438e6e13436b0190910ef7da49ce4f5a44f4)

Conflicts:
	crypto/blake2b_generic.c

(trivial conflict due to upstream commit 9332a9e73918,
"crypto: Replace HTTP links with HTTPS ones")

Bug: 178411248
Change-Id: I8b58cd0e2892a873866ead52fa4dba3edabdeeb3
Signed-off-by: Eric Biggers <ebiggers@google.com>