IPoIB: Fix unused variable warning

The conversion to use netdevice internal stats left an unused variable
in ipoib_neigh_free(), since there's no longer any reason to get
netdev_priv() in order to increment dropped packets.  Delete the
unused priv variable.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>