pst: improve shutdown_post_sync handler

It is desirable to shut down the raid controller even in the face of a
panic. In the SCHEDULER_STOPPED() case, set the interrupt mask bits so
that we request a polled wait, rather than sleep(), from
iop_queue_wait_msg().

Tweak the function name and signature.

Reviewed by: markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42337