Refine r301173 a bit.

Refine r301173 a bit.

- Don't execute any of g_mirror_shutdown_post_sync() when panicking. We
  cannot safely idle the mirror or stop synchronization in that state, and
  the current attempts to do so complicate debugging of gmirror itself.
- Check for a non-NULL panicstr instead of using SCHEDULER_STOPPED(). The
  latter was added for use in the locking primitives.

Reviewed by:	mav, pjd
MFC after:	2 weeks
Sponsored by:	Dell EMC Isilon