With EARLY_AP_STARTUP enabled, we are seeing crashes in softclock_call_cc()

With EARLY_AP_STARTUP enabled, we are seeing crashes in softclock_call_cc()
during bootup. Debugging information shows that softclock_call_cc() is
trying to execute the vt_consdev.vd_timer callout, and the callout
structure contains a NULL c_func.

This appears to be due to a race between vt_upgrade() running
callout_reset() and vt_resume_flush_timer() calling callout_schedule().

Fix the race by ensuring that vd_timer_armed is always set before
attempting to (re)schedule the callout.

Discussed with:	emaste
MFC after:	2 weeks
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D9828