cifs: call strtobool instead of custom implementation

Meanwhile it cleans up the code, the behaviour is slightly changed. In case of
providing non-boolean value it will fails with corresponding error. In the
original code the attempt of an update was just ignored in such case.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Jeff Layton <jlayton@poochiereds.net>
Reviewed-by: Alexander Bokovoy <ab@samba.org>
Signed-off-by: Steve French <steve.french@primarydata.com>