netfilter: nft_payload: do not update layer 4 checksum when mangling fragments

IP fragments do not come with the transport header, hence skip bogus
layer 4 checksum updates.

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