staging: comedi: fix extreme case of comedi_nsamples_left()

staging: comedi: fix extreme case of comedi_nsamples_left()

`comedi_nsamples_left(s, nsamples)` returns the number of samples
remaining to complete an asynchronous command or the passed in
`nsamples`, whichever is lower.  However, it goes wrong in the extreme
case of setting the `nsamples` parameter to `UINT_MAX` when the number
of conversions per "scan" (`s->async->cmd.scan_end_arg`) is 1.  It uses
`comedi_nscans_remaining(s, nscans)` to determine the number of scans
remaining, or the parameter `nscans`, whichever is lower.  To determine
the parameter `nscans`, it divides `nsamples` by the number of
conversions per scan and adds 1.  The addition of 1 is to avoid setting
the parameter `nscans` to 0, as `comedi_nscans_remaining(s, nscans)`
treats that value specially.  However in the extreme case where
`nsamples` is `UINT_MAX` and the number of samples per scan is 1, the
addition of 1 to `nscans` overflows, producing the unwanted 0.

Fix it by refactoring new a function `__comedi_nscans_remaining(s,
nscans)` out of `comedi_nscans_remaining(s, nscans)`.  The new function
does everything except the special handling when `nscans` is 0.  Change
`comedi_nsamples_remaining()` to call the new function without adding 1
to `nscans` to avoid the overflow.

This overflow bug doesn't affect any of the current COMEDI drivers.  I
stumbled across it while changing to one of the drivers.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>