vmxnet3: fix starving rx ring whenoc_skb kb fails

If the rx ring is completely empty, then the device may never fire an rx
interrupt. Unfortunately, the rx interrupt is what triggers populating the
rx ring with fresh buffers, so this will cause networking to lock up.

This patch replenishes the skb in recv descriptor as soon as it is
peeled off while processing rx completions. If the skb/buffer
allocation fails, existing one is recycled and the packet in hand is
dropped. This way none of the RX desc is ever left empty, thus avoiding
starvation

Signed-off-by: Scott J. Goldman <scottjg@vmware.com>
Signed-off-by: Shreyas N Bhatewara <sbhatewara@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>