device: fix WaitPool sync.Cond usage

device: fix WaitPool sync.Cond usage

The sync.Locker used with a sync.Cond must be acquired when changing
the associated condition, otherwise there is a window within
sync.Cond.Wait() where a wake-up may be missed.

Fixes: 4846070 ("device: use a waiting sync.Pool instead of a channel")
Reviewed-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Jordan Whited <jordan@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>