IB/iser: Fix module init not cleaning up on error flow

Destroy workqueue on transport register error, also
release kmem cache on workqueue allocation error.

Signed-off-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Doug Ledford <dledford@redhat.com>