iio:core: fix IIO_VAL_FRACTIONAL sign handling

7985e7c ("iio: Introduce a new fractional value type") introduced a
new IIO_VAL_FRACTIONAL value type meant to represent rational type numbers
expressed by a numerator and denominator combination.

Formating of IIO_VAL_FRACTIONAL values relies upon do_div() usage. This
fails handling negative values properly since parameters are reevaluated
as unsigned values.
Fix this by using div_s64_rem() instead. Computed integer part will carry
properly signed value. Formatted fractional part will always be positive.

Fixes: 7985e7c ("iio: Introduce a new fractional value type")
Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/industrialio-core.c

@@ -613,9 +613,8 @@ ssize_t iio_format_value(char *buf, unsigned int type, int size, int *vals)
 			return sprintf(buf, "%d.%09u\n", vals[0], vals[1]);
 	case IIO_VAL_FRACTIONAL:
 		tmp = div_s64((s64)vals[0] * 1000000000LL, vals[1]);
-		vals[1] = do_div(tmp, 1000000000LL);
-		vals[0] = tmp;
-		return sprintf(buf, "%d.%09u\n", vals[0], vals[1]);
+		vals[0] = (int)div_s64_rem(tmp, 1000000000, &vals[1]);
+		return sprintf(buf, "%d.%09u\n", vals[0], abs(vals[1]));
 	case IIO_VAL_FRACTIONAL_LOG2:
 		tmp = (s64)vals[0] * 1000000000LL >> vals[1];
 		vals[1] = do_div(tmp, 1000000000LL);