netlink_delinearize: support for bitfield payload statement with binary operation

netlink_delinearize: support for bitfield payload statement with binary operation

Add a new function to deal with payload statement delinearization with
binop expression.

Infer the payload offset from the mask, then walk the template list to
determine if estimated offset falls within a matching header field. If
so, then validate that this is not a raw expression but an actual
bitfield matching. Finally, trim the payload expression length
accordingly and adjust the payload offset.

instead of:

	@nh,8,5 set 0x0

it displays:

	ip dscp and 0x1

Update tests/py to cover for this enhancement.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>