If a drive has gone down and has dirty buffers associated with it,
we'll get a panic when we try to vn_close it. Check for this
situation and discard any buffers; they're toast anyway.
Only complain about usage count if DEBUG_WARNINGS is set.
check_drive:
Change parameter name from drivename to devicename.
Get the check for a referenced drive right.
If the partition isn't a vinum drive, set the last error to ENODEV.
vinum_scandisk:
Change parameter name from drivename [] to devicename [].