[PPPOL2TP]: Fix SMP issues in skb reorder queue handling

When walking a session's packet reorder queue, use
skb_queue_walk_safe() since the list could be modified inside the
loop.

Rearrange the unlinking skbs from the reorder queue such that it is
done while the queue lock is held in pppol2tp_recv_dequeue() when
walking the skb list.

A version of this patch was suggested by Jarek Poplawski.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>