infiniband: ipoib: Sanitize neighbour handling in ipoib_main.c

Reduce the number of dst_get_neighbour_noref() calls within a single
call chain.  Primarily by passing the neighbour pointer down to the
helper functions.

Handle dst_get_neighbour_noref() returning NULL in ipoib_start_xmit()
by incrementing the dropped counter and freeing the packet.  We don't
want it to fall through into the ARP/RARP/multicast handling, since
that should only happen when skb_dst() is NULL.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Roland Dreier <roland@purestorage.com>