b44: Use dev_kfree_skb_irq() in b44_tx()

Reported issues when using dev_kfree_skb() on UP systems and
systems with low numbers of cores.  dev_kfree_skb_irq() will
properly save IRQ state before freeing the skb.

Tested on 3.1.1 and 3.2_rc2

Example of reproducible trace of kernel 3.1.1
------------[ cut here ]------------
   WARNING: at kernel/softirq.c:159 local_bh_enable+0x32/0x79()
   ...
   Pid: 0, comm: swapper Not tainted 3.1.1-gentoo #1
   Call Trace:
    [<c1022970>] warn_slowpath_common+0x65/0x7a
    [<c102699e>] ? local_bh_enable+0x32/0x79
    [<c1022994>] warn_slowpath_null+0xf/0x13
    [<c102699e>] local_bh_enable+0x32/0x79
    [<c134bfd8>] destroy_conntrack+0x7c/0x9b
    [<c134890b>] nf_conntrack_destroy+0x1f/0x26
    [<c132e3a6>] skb_release_head_state+0x74/0x83
    [<c132e286>] __kfree_skb+0xb/0x6b
    [<c132e30a>] consume_skb+0x24/0x26
    [<c127c925>] b44_poll+0xaa/0x449
    [<c1333ca1>] net_rx_action+0x3f/0xea
    [<c1026a44>] __do_softirq+0x5f/0xd5
    [<c10269e5>] ? local_bh_enable+0x79/0x79
    <IRQ>  [<c1026c32>] ? irq_exit+0x34/0x8d
    [<c1003628>] ? do_IRQ+0x74/0x87
    [<c13f5329>] ? common_interrupt+0x29/0x30
    [<c1006e18>] ? default_idle+0x29/0x3e
    [<c10015a7>] ? cpu_idle+0x2f/0x5d
    [<c13e91c5>] ? rest_init+0x79/0x7b
    [<c15c66a9>] ? start_kernel+0x297/0x29c
    [<c15c60b0>] ? i386_start_kernel+0xb0/0xb7
   ---[ end trace 583f33bb1aa207a9 ]---
Signed-off-by: Xander Hover <LKML@hover.be>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/b44.c

@@ -608,7 +608,7 @@ static void b44_tx(struct b44 *bp)
 				 skb->len,
 				 DMA_TO_DEVICE);
 		rp->skb = NULL;
-		dev_kfree_skb(skb);
+		dev_kfree_skb_irq(skb);
 	}
 
 	bp->tx_cons = cons;