cam/iosched: Pass in the disk when initializing

[imp]

Summary

The disk is nice to have at times, especially when you need the sector
size. At present, the only plans for this are related to logging.

Sponsored by: Netflix

[imp]

Misc updates with testing

[jhb]

sys/cam/cam_iosched.c

1161

Old bug: this would be more readable IMO with a local variable, that is:

{
     struct cam_iosched_softc *isc;

     isc = malloc(sizeof(*isc), ...);
     ...
     *iscp = isc;
     return 0;
}