USB: fix thread-unsafe anchor utiliy routines

This patch fixes a race condition in two utility routines
related to the removal/unlinking of urbs from an anchor.

If two threads are concurrently accessing the same anchor,
both could end up with the same urb - thinking they are
the exclusive owner.

Alan Stern pointed out a related issue in
usb_unlink_anchored_urbs:

"The URB isn't removed from the anchor until it completes
 (as a by-product of completion, in fact), which might not
 be for quite some time after the unlink call returns.
 In the meantime, the subroutine will keep trying to unlink
 it, over and over again."

Cc: stable <stable@kernel.org>
Cc: Oliver Neukum <oneukum@suse.de>
Cc: Greg Kroah-Hartman <greg@kroah.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>