82596: do not panic on out of memory

If dev_alloc_skb() failed then free already allocated skbs.
remove_rx_bufs() can be called multiple times, so set rbd->skb to NULL
to avoid double free. remove_rx_bufs() was moved upwards to be seen by
init_rx_bufs().

Signed-off-by: Kulikov Vasiliy <segooon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>