iscsi: adjust shutdown_pre_sync handler

Don't attempt to service reconnections if RB_NOSYNC is set. More
crucially, don't do it if the scheduler is stopped, as the maintenance
thread will never run again.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42342