MFC r342639:
When allocating a new keyboard at vt_upgrade() time, unwind any cngrabs
done on the old keyboard and then do the corresponding number of grabs
on the new keyboard.
This fixes a race that can leave the system with a non-functioning
keyboard. It goes like this...
- The bios claims there is an AT keyboard, atkbd attaches.
- SI_SUB_INT_CONFIG_HOOKS runs.
- USB probes devices. Devices begin attaching, including disks.
- GELI prompts for a password for a just-attached disk, which results
in a cngrab() while atkbd is the keyboard.
- A USB keyboard attaches.
- vt_upgrade() runs and switches the keyboard to the new USB keyboard,
but because cngrab was never called for it, it's not activated and
keystrokes are ignored.
- Now there is no functional keyboard and no way to get one; even
plugging in a different USB keyboard doesn't help, because the console
is still grabbed, still waiting for a GELI pw.
UnifiedSplitRaw
