watchdog: cpwd: use generic compat_ptr_ioctl

The cpwd_compat_ioctl() contains a bogus mutex that dates
back to a leftover BKL instance.

Simplify the implementation by using the new compat_ptr_ioctl()
helper function that will do the right thing for all calls
here.

Note that WIOCSTART/WIOCSTOP don't take any arguments, so
the compat_ptr() conversion is not needed here, but it also
doesn't hurt.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20190814204259.120942-6-arnd@arndb.de


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>