cam: Add probes for xpt actions

cam::xpt:action(union ccb *)
cam::xpt:done((union ccb *)
cam::xpt:async-cb(void *cbarg, uint32_t async_code, struct cam_path
*path, void *async_arg);

Called when xpt_action(), xpt_done*() and the xpt async callbacks are
called.

Sponsored by: Netflix
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D54469