net/usbnet: avoid recursive locking in usbnet_stop()

|kernel BUG at kernel/rtmutex.c:724!
|[<c029599c>] (rt_spin_lock_slowlock+0x108/0x2bc) from [<c01c2330>] (defer_bh+0x1c/0xb4)
|[<c01c2330>] (defer_bh+0x1c/0xb4) from [<c01c3afc>] (rx_complete+0x14c/0x194)
|[<c01c3afc>] (rx_complete+0x14c/0x194) from [<c01cac88>] (usb_hcd_giveback_urb+0xa0/0xf0)
|[<c01cac88>] (usb_hcd_giveback_urb+0xa0/0xf0) from [<c01e1ff4>] (musb_giveback+0x34/0x40)
|[<c01e1ff4>] (musb_giveback+0x34/0x40) from [<c01e2b1c>] (musb_advance_schedule+0xb4/0x1c0)
|[<c01e2b1c>] (musb_advance_schedule+0xb4/0x1c0) from [<c01e2ca8>] (musb_cleanup_urb.isra.9+0x80/0x8c)
|[<c01e2ca8>] (musb_cleanup_urb.isra.9+0x80/0x8c) from [<c01e2ed0>] (musb_urb_dequeue+0xec/0x108)
|[<c01e2ed0>] (musb_urb_dequeue+0xec/0x108) from [<c01cbb90>] (unlink1+0xbc/0xcc)
|[<c01cbb90>] (unlink1+0xbc/0xcc) from [<c01cc2ec>] (usb_hcd_unlink_urb+0x54/0xa8)
|[<c01cc2ec>] (usb_hcd_unlink_urb+0x54/0xa8) from [<c01c2a84>] (unlink_urbs.isra.17+0x2c/0x58)
|[<c01c2a84>] (unlink_urbs.isra.17+0x2c/0x58) from [<c01c2b44>] (usbnet_terminate_urbs+0x94/0x10c)
|[<c01c2b44>] (usbnet_terminate_urbs+0x94/0x10c) from [<c01c2d68>] (usbnet_stop+0x100/0x15c)
|[<c01c2d68>] (usbnet_stop+0x100/0x15c) from [<c020f718>] (__dev_close_many+0x94/0xc8)

defer_bh() takes the lock which is hold during unlink_urbs(). The safe
walk suggest that the skb will be removed from the list and this is done
by defer_bh() so it seems to be okay to drop the lock here.

Cc: stable@kernel.org
Reported-by: Aníbal Almeida Pinto <anibal.pinto@efacec.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: David S. Miller <davem@davemloft.net>