Two new helpers: scsi_device_get and scsi_device_put that get/release a
reference to the underlying HBA driver and increment/decrement
->access_count.  Cleanup ->attach/->detach routines in the upper layer
drivers a bit to consolidate the error pathes once we're cleaning them
up.

->attach and ->detach in upper layer drivers are mandatory now (not
having them would be rather pointless).

(note that the sd.c changes are not in this patch, it'll be part of
my next, bigger patch)