Add a witness check to enforce that no non-sleeping locks are held when
they shouldn't be.

I used this during driver bring-up to find that the Linux driver holds a
whole lot of locks whilst doing their equivalent of busdma operations.

If this works out well, it should be added to the other architecture busdma
implementations to aid in similar debugging.

Tested:

• bounce buffer and dmar busdma, Lenovo X230 laptop, all the internal hardware
• ath(4) too

Discussed with: jhb