MFC r304153:

Quick fix for locking fixes in r172250. The lock added there was per-
virtual-device, but needs to be per-physical-device so that it protects
shared data. Usually, scp->sc->write_in_progress got corrupted first
and further corruption was limited when this variable was left at nonzero
with no write in progress.

Attempt to fix missing lock destruction in r162285. Put it with the
lock destruction for r172250 after moving the latter. Both might be
unreachable.

To demonstrate the bug, find a buggy syscall or sysctl that calls
printf(9) and run this often. Run hd /dev/zero >/dev/ttyvN for any
N != 0. The console spam goes to ttyv0 and the non-console spam goes
to ttyvN, so the lock provided no protection (but it helped for
N == 0).