USB changed behaviour of unlink_urb so that it gives warning and backtrace when
being used synchronously. The correct current behaviour is to us kill_urb
in that case.

Original patch from Stephen Hemminger
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: Jean Tourrilhes <jt@hpl.hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>