proto: use NFT_PAYLOAD_L4CSUM_PSEUDOHDR flag to mangle UDP checksum

proto: use NFT_PAYLOAD_L4CSUM_PSEUDOHDR flag to mangle UDP checksum

There are two mechanisms to update the UDP checksum field:

 1) _CSUM_TYPE and _CSUM_OFFSET which specify the type of checksum
    (e.g. inet) and offset where it is located.
 2) use NFT_PAYLOAD_L4CSUM_PSEUDOHDR flag to use layer 4 kernel
    protocol parser.

The problem with 1) is that it is inconditional, that is, csum_type and
csum_offset cannot deal with zero UDP checksum.

Use NFT_PAYLOAD_L4CSUM_PSEUDOHDR flag instead since it relies on the
layer 4 kernel parser which skips updating zero UDP checksum.

Extend test coverage for the UDP mangling with and without zero
checksum.

Fixes: e6c9174e13b2 ("proto: add checksum key information to struct proto_desc")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>