USB: don't free bandwidth_mutex too early

The USB core contains a bug that can show up when a USB-3 host
controller is removed.  If the primary (USB-2) hcd structure is
released before the shared (USB-3) hcd, the core will try to do a
double-free of the common bandwidth_mutex.

The problem was described in graphical form by Chung-Geol Kim, who
first reported it:

=================================================
     At *remove USB(3.0) Storage
     sequence <1> --> <5> ((Problem Case))
=================================================
                                  VOLD
------------------------------------|------------
                                 (uevent)
                            ________|_________
                           |<1>               |
                           |dwc3_otg_sm_work  |
                           |usb_put_hcd       |
                           |peer_hcd(kref=2)|
                           |__________________|
                            ________|_________
                           |<2>               |
                           |New USB BUS #2    |
                           |                  |
                           |peer_hcd(kref=1)  |
                           |                  |
                         --(Link)-bandXX_mutex|
                         | |__________________|
                         |
    ___________________  |
   |<3>                | |
   |dwc3_otg_sm_work   | |
   |usb_put_hcd        | |
   |primary_hcd(kref=1)| |
   |___________________| |
    _________|_________  |
   |<4>                | |
   |New USB BUS #1     | |
   |hcd_release        | |
   |primary_hcd(kref=0)| |
   |                   | |
   |bandXX_mutex(free) |<-
   |___________________|
                               (( VOLD ))
                            ______|___________
                           |<5>               |
                           |      SCSI        |
                           |usb_put_hcd       |
                           |peer_hcd(kref=0)  |
                           |*hcd_release      |
                           |bandXX_mutex(free*)|<- double free
                           |__________________|

=================================================

This happens because hcd_release() frees the bandwidth_mutex whenever
it sees a primary hcd being released (which is not a very good idea
in any case), but in the course of releasing the primary hcd, it
changes the pointers in the shared hcd in such a way that the shared
hcd will appear to be primary when it gets released.

This patch fixes the problem by changing hcd_release() so that it
deallocates the bandwidth_mutex only when the _last_ hcd structure
referencing it is released.  The patch also removes an unnecessary
test, so that when an hcd is released, both the shared_hcd and
primary_hcd pointers in the hcd's peer will be cleared.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-by: Chung-Geol Kim <chunggeol.kim@samsung.com>
Tested-by: Chung-Geol Kim <chunggeol.kim@samsung.com>
CC: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>