IPoIB: Recycle loopback skbs instead of freeing and reallocating

InfiniBand HCAs replicate multicast packets back to the QP that sent
them if that QP is attached to the destination multicast group.  This
means that IPoIB multicasts are often replicated back to the receive
queue of the interface that generated them.  To avoid confusing the
network stack, we drop these duplicates within the IPoIB driver.

However, there's no reason to free the skb that received the duplicate
and then immediately allocate a new skb to post to the receive queue.
We can be more efficient and just repost the same skb.

Signed-off-by: Roland Dreier <rolandd@cisco.com>