kdf/argon2: use BLAKE2b hash_buffers function instead of _gcry_md_*

* cipher/kdf.c (argon2_fill_first_blocks): Convert to use iov
hash_buffers API instead of _gcry_md_*.

More direct use of BLAKE2b avoids overhead from md object creation
and cleanup.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>