`comedi_event()` is called from low-level drivers to handle comedi
asynchronous command event flags.  As a safety check, it checks the
subdevice's "run" flags to make sure an asynchronous command is running.
It can also change the run flags to mark the command as no longer
running (possibly also marking it as terminated with an error).
Checking the runflags and modifying them involves two uses of the
subdevice's spin-lock.  It seems better to do it with a single use of
the spin-lock.  This also avoids possible interactions with
`do_become_nonbusy()`.

Acquire the subdevice's spin-lock at the start of `comedi_event()` and
release it near the end, before a possible call to `kill_fasync()` (but
after it's parameter values have been determined).

Add and make use of few new inline helper functions:

* `__comedi_clear_subdevice_runflags()` -- clears some run flags without
  using the spin-lock
* `__comedi_set_subdevice_runflags()` -- sets some run flags without
  using the spin-lock
* `__comedi_get_subdevice_runflags()` -- a spin-lockless version of
  `comedi_get_subdevice_runflags()
* `__comedi_is_subdevice_running()` -- a spin-lockless version of
* `comedi_is_subdevice_running()`

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>