crypto: dh - fix calculating encoded key size

It was forgotten to increase DH_KPP_SECRET_MIN_SIZE to include 'q_size',
causing an out-of-bounds write of 4 bytes in crypto_dh_encode_key(), and
an out-of-bounds read of 4 bytes in crypto_dh_decode_key().  Fix it, and
fix the lengths of the test vectors to match this.

Reported-by:  <syzbot+6d38d558c25b53b8f4ed@syzkaller.appspotmail.com>
Fixes: e3fe0ae1 ("crypto: dh - add public key verification test")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>