netfilter: nft_payload: add missing attribute validation for payload csum flags

Add missing attribute validation for NFTA_PAYLOAD_CSUM_FLAGS
to the netlink policy.

Fixes: 18140969 ("netfilter: nft_payload: layer 4 checksum adjustment for pseudoheader fields")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>