disables tag queuing temporarily in order to allow controllers a window
to safely perform transfer negotiation with non-compliant devices. Before
this change, CAM would restore the queue depth to the controller specified
maximum or device quirk level rather than any depth determined by reactions
to QUEUE FULL/BUSY events or an explicit user setting.
During device probe, initialize the flags field for XPT_SCAN_BUS.
The uninitialized value often confused CAM into not bothering to
issue an AC_FOUND_DEVICE async event for new devices. The reason
this bug wasn't reported earlier is that CAM manually announces
devices after the initial system bus scans.
MFC: 3 days
df8f908079