netfilter: bitwise: add support for doing AND, OR and XOR directly

Hitherto, these operations have been converted in user space to
mask-and-xor operations on one register and two immediate values, and it
is the latter which have been evaluated by the kernel.  We add support
for evaluating these operations directly in kernel space on one register
and either an immediate value or a second register.

Pablo made a few changes to the original patch:

- EINVAL if NFTA_BITWISE_SREG2 is used with fast version.
- Allow _AND,_OR,_XOR with _DATA != sizeof(u32)
- Dump _SREG2 or _DATA with _AND,_OR,_XOR

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