netfilter: bitwise: add NFTA_BITWISE_OP netlink attribute.

Add a new bitwise netlink attribute, NFTA_BITWISE_OP, which is set to a
value of a new enum, nft_bitwise_ops.  It describes the type of
operation an expression contains.  Currently, it only has one value:
NFT_BITWISE_BOOL.  More values will be added later to implement shifts.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>