netfilter: bitwise: remove NULL comparisons from attribute checks.

In later patches, we will be adding more checks.  In order to be
consistent and prevent complaints from checkpatch.pl, replace the
existing comparisons with NULL with logical NOT operators.

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