netfilter: nft_payload: mangle ckecksum if NFT_PAYLOAD_L4CSUM_PSEUDOHDR is set

If the NFT_PAYLOAD_L4CSUM_PSEUDOHDR flag is set, then mangle layer 4
checksum. This should not depend on csum_type NFT_PAYLOAD_CSUM_INET
since IPv6 header has no checksum field, but still an update of any of
the pseudoheader fields may trigger a layer 4 checksum update.

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