V4L/DVB (7321): pvrusb2: Rework context handling and initialization

V4L/DVB (7321): pvrusb2: Rework context handling and initialization

This change significantly rearranges pvr2_context level initialization
and operation:

1. A new kernel thread is set up for management of the context.

2. Destruction of the pvr2_context instance is moved into the kernel
   thread.  No other context is able to remove the instance; doing
   this simplifies lock handling.

3. The callback into pvrusb2-main, which is used to trigger
   initialization of each interface, is now issued from this kernel
   thread.  Previously it had been indirectly issued out of the work
   queue thread in pvr2_hdw, which led to deadlock issues if the
   interface needed to change a control setting (which in turn
   requires dispatch of another work queue entry).

4. Callbacks into the interfaces (via the pvr2_channel structure) are
   now issued strictly from this thread.  The net result of this is
   that such callback functions can now also safely operate driver
   controls without deadlocking the work queue.  (At the moment this
   is not actually a problem, but I'm anticipating issues with this in
   the future).

5. There is no longer any need for anyone to enter / exit the
   pvr2_context structure.  Implementation of the kernel thread here
   allows this all to be internal now, simplifying other logic.

6. A very very longstanding issue involving a mutex deadlock between
   the pvrusb2 driver and v4l should now be solved.  The deadlock
   involved the pvr2_context mutex and a globals-protecting mutex in
   v4l.  During initialization the driver would take the pvr2_context
   mutex first then the v4l2 interface would register with v4l and
   implicitly take the v4l mutex.  Later when v4l would call back into
   the driver, the two mutexes could possibly be taken in the opposite
   order, a situation that can lead to deadlock.  In practice this
   really wasn't an issue unless a v4l app tried to start VERY early
   after the driver appeared.  However it still needed to be solved,
   and with the use of the kernel thread relieving need for
   pvr2_context mutex, the problem should be finally solved.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>