IPoIB: Free ipoib neigh on path record failure so path rec queries are retried

If IPoIB fails to look up a path record (eg if it tries during an SM
failover when one SM is dead but the new one hasn't taken over yet), the
driver ends up with a neighbour structure but no address handle (AH).
There's no mechanism to recover from this: any further packets sent to
this destination will be silently dumped in ipoib_start_xmit().

Fix this by freeing the neighbour structures when a path rec query
fails, so that the next packet queued to be sent will trigger a new path
record query.

Signed-off-by: Roland Dreier <roland@purestorage.com>