iscsi: Mark iSCSI CAM sims as non-pollable.

Previously, iscsi_poll() just panicked. This meant if you got a panic
on a box when using the iSCSI initiator, the attempt to shutdown would
trigger a nested panic and never write out a core. Now, CCB's sent to
iSCSI devices (such as the sychronize-cache request in dashutdown())
just fail with a timeout during a panic shutdown.

Sponsored by: Chelsio

(cherry picked from commit 47769bc5573fc5d2263f9e20cb06f6c12d0a82e7)