netfilter: nf_tables: Enable fast nft_cmp for inverted matches

Add a boolean indicating NFT_CMP_NEQ. To include it into the match
decision, it is sufficient to XOR it with the data comparison's result.

While being at it, store the mask that is calculated during expression
init and free the eval routine from having to recalculate it each time.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>