netfilter: nft_compat: set IP6T_F_PROTO flag if protocol is set

[ Upstream commit 749177ccc74f9c6d0f51bd78a15c652a2134aa11 ]

ip6tables extensions check for this flag to restrict match/target to a
given protocol. Without this flag set, SYNPROXY6 returns an error.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>