genl_magic: Resolve logical-op warnings

Resolve "logical 'and' applied to non-boolean constant" warnings"
that appear in W=2 builds by adding !! to a bit test.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>