Restore the CAM XPT peripheral generation counter, and export it via sysctl.

Define it as an atomic uint32_t. These increments happen infrequently
enough for the atomic overhead to be a problem, and since they're now
independent atomics, they won't contend with xpt_lock_buses().

This counter is useful as a means of cheaply identifying whether any changes
have been made to the CAM peripheral list. Userland programs have no guarantee
that the counter won't change on them while being returned or while processing
the information, so they must be written accordingly.

Discussed with: ken, mav (in general)
MFC after: 1 week
Sponsored by: Spectra Logic