IPoIB: Fix skb leak when freeing neighbour

ipoib_neigh_free() is sometimes called while neighbour is still alive,
so it might still have queued skbs.  Fix skb leak in this case.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>