xen-netfront: Rework the fix for Rx stall during OOM and network stress

The commit 90c311b0 ("xen-netfront: Fix Rx stall during network
stress and OOM") caused the refill timer to be triggerred almost on
all invocations of xennet_alloc_rx_buffers for certain workloads.
This reworks the fix by reverting to the old behaviour and taking into
consideration the skb allocation failure. Refill timer is now triggered
on insufficient requests or skb allocation failure.

Signed-off-by: Vineeth Remanan Pillai <vineethp@amazon.com>
Fixes: 90c311b0 (xen-netfront: Fix Rx stall during network stress and OOM)
Reported-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>