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

commit 9d6effb2f1523eb84516e44213c00f2fd9e6afff upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>