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

(cherry picked from commit c4dacfa7f4b82f23ec0924d9db772860b2066f9b)
(cherry picked from commit f97aab79868cd7d891c52b14bd964523fa56f015)