Secondary FAT volumes get excusilvely locked on boot

After r75772, fixing open handles counting in VCB, FSCTL for un/locking volumes can work now.

That means that vfatlib, on chkdsk on a secondary FAT volume will attempt to unmount the volume after check.

It seems that the volume is unreachable to ROS after that. It is either improperly unmounted (likely) or remounted raw (unlikely).