cxgb3: Fix kernel crash caused by uninitialized l2t_entry.arpq

Commit 147e70e6

 ("cxgb3: Use SKB list interfaces instead of home-grown
implementation.") causes a crash in t3_l2t_send_slow() when an iWARP
connection request is received.  This is because the new l2t_entry.arpq
skb queue is never initialized, and therefore trying to add an skb to
it causes a NULL dereference.  With the old code there was no need to
initialize the queues because the l2t_entry structures were zeroed,
and the code used NULL to mean empty.

Fix this by adding __skb_queue_head_init() when all the l2t_entry
structures get allocated.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>