Rename _cscan_atomic.h and _cscan_bus.h to atomic_san.h and bus_san.h

Other kernel sanitizers (KMSAN, KASAN) require interceptors as well, so
put these in a more generic place as a step towards importing the other
sanitizers.

No functional change intended.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29103