Fix panic on shutdown due to iscsi event priority

iscsi's shutdown_pre_sync prio was SHUTDOWN_PRI_FIRST which caused it to
run before other high priority handlers such as filesystems e.g. ZFS.

This meant the iscsi sessions where removed before the ZFS geom consumer
was closed, resulting in a panic from g_access calls on debug kernels
due to negative acr.

Instead use the same as the old iscsi_initiator SHUTDOWN_PRI_DEFAULT-1
which allows it to run before dashutdown etc but after filesystems.

MFC after: 2 weeks
Sponsored by: Multiplay