netfilter: bitwise: add NFTA_BITWISE_DATA attribute.

Add a new bitwise netlink attribute that will be used by shift
operations to store the size of the shift.  It is not used by boolean
operations.

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