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