[IPoIB] Drop RX packets when out of memory

Change the way IPoIB handles RX packets when it can't allocate a new
receive skbuff.  If the allocation of a new receive skb fails, we now
drop the packet we just received and repost the original receive skb.
This means that the receive ring always stays full and we don't have
to monkey around with trying to schedule a refill task for later.

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