IPoIB: Fix oops if xmit is called when priv->broadcast is NULL

If a port goes down, ipoib_ib_dev_down() is invoked -- which flushes
the mcasts (clearing priv->broadcast) and clearing the path record
cache.  If ipoib_start_xmit() is then invoked (before the broadcast
group is rejoined), a kernel oops results from attempting to access
priv->broadcast, which is still unset.

Returning NULL from path_rec_create() if priv->broadcast is NULL is a
harmless way of bypassing the problem -- the offending packet is
simply discarded "without prejudice."

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>