Fix intermittent hangs in xgmac_rx_refill. If a ring buffer entry already
had an skb allocated, then xgmac_rx_refill would get stuck in a loop. This
can happen on a rx error when we just leave the skb allocated to the entry.

[ 7884.510000] INFO: rcu_preempt detected stall on CPU 0 (t=727315 jiffies)
[ 7884.510000] [<c0010a59>] (unwind_backtrace+0x1/0x98) from [<c006fd93>] (__rcu_pending+0x11b/0x2c4)
[ 7884.510000] [<c006fd93>] (__rcu_pending+0x11b/0x2c4) from [<c0070b95>] (rcu_check_callbacks+0xed/0x1a8)
[ 7884.510000] [<c0070b95>] (rcu_check_callbacks+0xed/0x1a8) from [<c0036abb>] (update_process_times+0x2b/0x48)
[ 7884.510000] [<c0036abb>] (update_process_times+0x2b/0x48) from [<c004e8fd>] (tick_sched_timer+0x51/0x94)
[ 7884.510000] [<c004e8fd>] (tick_sched_timer+0x51/0x94) from [<c0045527>] (__run_hrtimer+0x4f/0x1e8)
[ 7884.510000] [<c0045527>] (__run_hrtimer+0x4f/0x1e8) from [<c0046003>] (hrtimer_interrupt+0xd7/0x1e4)
[ 7884.510000] [<c0046003>] (hrtimer_interrupt+0xd7/0x1e4) from [<c00101d3>] (twd_handler+0x17/0x24)
[ 7884.510000] [<c00101d3>] (twd_handler+0x17/0x24) from [<c006be39>] (handle_percpu_devid_irq+0x59/0x114)
[ 7884.510000] [<c006be39>] (handle_percpu_devid_irq+0x59/0x114) from [<c0069aab>] (generic_handle_irq+0x17/0x2c)
[ 7884.510000] [<c0069aab>] (generic_handle_irq+0x17/0x2c) from [<c000cc8d>] (handle_IRQ+0x35/0x7c)
[ 7884.510000] [<c000cc8d>] (handle_IRQ+0x35/0x7c) from [<c033b153>] (__irq_svc+0x33/0xb8)
[ 7884.510000] [<c033b153>] (__irq_svc+0x33/0xb8) from [<c0244b06>] (xgmac_rx_refill+0x3a/0x140)
[ 7884.510000] [<c0244b06>] (xgmac_rx_refill+0x3a/0x140) from [<c02458ed>] (xgmac_poll+0x265/0x3bc)
[ 7884.510000] [<c02458ed>] (xgmac_poll+0x265/0x3bc) from [<c029fcbf>] (net_rx_action+0xc3/0x200)
[ 7884.510000] [<c029fcbf>] (net_rx_action+0xc3/0x200) from [<c0030cab>] (__do_softirq+0xa3/0x1bc)

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: David S. Miller <davem@davemloft.net>