reboot: Avoid unlocking Giant if the scheduler is stopped

[markj]

Summary

When the scheduler is stopped, mtx_unlock() turns into a no-op, so the
loop

    while (mtx_owned(&Giant))
	    mtx_unlock(&Giant);

runs forever if the calling thread has Giant locked.

[mhorne]

No objection, but I wonder why we loop at all.

[markj]

In D42460#968986, @mhorne wrote:

No objection, but I wonder why we loop at all.

Giant is recursive, so it could in principle be held multiple times by the same thread.

[imp]

I thought I'd hit accept yesterday this is fine. Crashes are super weird. I can't think of any other place we do this.. Giant really is special.