comedi: Use reference count for asynchronous command functions

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.

Change those functions in the Comedi core to use this pattern: if
`comedi_get_is_subdevice_running(s)` returns `true` then call a safe
version of the function with the same name prefixed with an underscore,
followed by a call to `comedi_put_is_subdevice_running(s)`, otherwise
take some default action.

`comedi_get_is_subdevice_running(s)` returning `true` ensures that the
details of the asynchronous command will not be destroyed before the
matching call to `comedi_put_is_subdevice_running(s)`.

Replace calls to those functions from elsewhere in the Comedi core with
calls to the safe versions of the functions.

The modified functions are: `comedi_buf_read_alloc()`,
`comedi_buf_read_free()`, `comedi_buf_read_n_available()`,
`comedi_buf_read_samples()`, `comedi_buf_write_alloc()`,
`comedi_buf_write_free()`, `comedi_buf_write_samples()`,
`comedi_bytes_per_scan()`, `comedi_event()`, `comedi_handle_events()`,
`comedi_inc_scan_progress()`, `comedi_nsamples_left()`,
`comedi_nscans_left()`.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://patch.msgid.link/20251023133001.8439-3-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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