mv643xx_eth: fix double add_timer() on the receive oom timer

Commit 12e4ab79 ("mv643xx_eth: be
more agressive about RX refill") changed the condition for the receive
out-of-memory timer to be scheduled from "the receive ring is empty"
to "the receive ring is not full".

This can lead to a situation where the receive out-of-memory timer is
pending because a previous rxq_refill() didn't manage to refill the
receive ring entirely as a result of being out of memory, and
rxq_refill() is then called again as a side effect of a packet receive
interrupt, and that rxq_refill() call then again does not succeed to
refill the entire receive ring with fresh empty skbuffs because we are
still out of memory, and then tries to call add_timer() on the already
scheduled out-of-memory timer.

This patch fixes this issue by changing the add_timer() call in
rxq_refill() to a mod_timer() call.  If the OOM timer was not already
scheduled, this will behave as before, whereas if it was already
scheduled, this patch will push back its firing time a bit, which is
safe because we've (unsuccessfully) attempted to refill the receive
ring just before we do this.
Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>

drivers/net/mv643xx_eth.c

@@ -509,10 +509,8 @@ static void rxq_refill(struct rx_queue *rxq)
 		skb_reserve(skb, 2);
 	}
 
-	if (rxq->rx_desc_count != rxq->rx_ring_size) {
-		rxq->rx_oom.expires = jiffies + (HZ / 10);
-		add_timer(&rxq->rx_oom);
-	}
+	if (rxq->rx_desc_count != rxq->rx_ring_size)
+		mod_timer(&rxq->rx_oom, jiffies + (HZ / 10));
 
 	spin_unlock_irqrestore(&mp->lock, flags);
 }