IB/iser: Release connection resources on RDMA_CM_EVENT_DEVICE_REMOVAL event

When a RDMA_CM_EVENT_DEVICE_REMOVAL event is raised, iSER should
release the connection resources.

This is necessary when the IB HCA module is unloaded while open-iscsi
is still running.  Currently, iSER just BUG()s.

Signed-off-by: Erez Zilber <erezz@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>