nft_set_pipapo: Fix bucket load in AVX2 lookup routine for six 8-bit groups

The sixth byte of packet data has to be looked up in the sixth group,
not in the seventh one, even if we load the bucket data into ymm6
(and not ymm5, for convenience of tracking stalls).

Without this fix, matching on a MAC address as first field of a set,
if 8-bit groups are selected (due to a small set size) would fail,
that is, the given MAC address would never match.

Reported-by: Nikita Yushchenko <nikita.yushchenko@virtuozzo.com>
Cc: <stable@vger.kernel.org> # 5.6.x
Fixes: 7400b063

 ("nft_set_pipapo: Introduce AVX2-based lookup implementation")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Tested-By: Nikita Yushchenko <nikita.yushchenko@virtuozzo.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>