comedi: Add reference counting for Comedi command handling

For interrupts from badly behaved hardware (as emulated by Syzbot), it
is possible for the Comedi core functions that manage the progress of
asynchronous data acquisition to be called from driver ISRs while no
asynchronous command has been set up, which can cause problems such as
invalid pointer dereferencing or dividing by zero.

To help protect against that, introduce new functions to maintain a
reference counter for asynchronous commands that are being set up.
`comedi_get_is_subdevice_running(s)` will check if a command has been
set up on a subdevice and is still marked as running, and if so will
increment the reference counter and return `true`, otherwise it will
return `false` without modifying the reference counter.
`comedi_put_is_subdevice_running(s)` will decrement the reference
counter and set a completion event when decremented to 0.

Change the `do_cmd_ioctl()` function (responsible for setting up the
asynchronous command) to  reinitialize the completion event and set the
reference counter to 1 before it marks the subdevice as running.  Change
the `do_become_nonbusy()` function (responsible for destroying a
completed command) to call `comedi_put_is_subdevice_running(s)` and wait
for the completion event after marking the subdevice as not running.

Because the subdevice normally gets marked as not running before the
call to `do_become_nonbusy()` (and may also be called when the Comedi
device is being detached from the low-level driver), add a new flag
`COMEDI_SRF_BUSY` to the set of subdevice run-flags that indicates that
an asynchronous command was set up and will need to be destroyed.  This
flag is set by `do_cmd_ioctl()` and cleared and checked by
`do_become_nonbusy()`.

Subsequent patches will change the Comedi core functions that are called
from low-level drivers for asynchrous command handling to make use of
the `comedi_get_is_subdevice_running()` and
`comedi_put_is_subdevice_running()` functions, and will modify the ISRs
of some of these low-level drivers if they dereference the subdevice's
`async` pointer directly.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://patch.msgid.link/20251023133001.8439-2-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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