IB/ipoib: change init sequence ordering

In preparation for using per device work queues, we need to move the
start of the neighbor thread task to after ipoib_ib_dev_init and move
the destruction of the neighbor task to before ipoib_ib_dev_cleanup.
Otherwise we will end up freeing our workqueue with work possibly
still on it.

Signed-off-by: Doug Ledford <dledford@redhat.com>