Expand ktr_mask to be a 64-bit unsigned integer.

The mask does not really need to be updated with atomic operations and
the downside of losing races during transitions is not great (it is
not marked volatile, so those races are pretty wide open as it is).

Differential Revision: https://reviews.freebsd.org/D2595
Reviewed by: emaste, neel, rpaulo
MFC after: 2 weeks