rtc: Fix overflow when converting time64_t to rtc_time

BugLink: https://bugs.launchpad.net/bugs/1826212

commit 36d46cdb upstream.

If we convert one large time values to rtc_time, in the original formula
'days * 86400' can be overflowed in 'unsigned int' type to make the formula
get one incorrect remain seconds value. Thus we can use div_s64_rem()
function to avoid this situation.
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/rtc/rtc-lib.c

@@ -52,13 +52,11 @@ EXPORT_SYMBOL(rtc_year_days);
  */
 void rtc_time64_to_tm(time64_t time, struct rtc_time *tm)
 {
-	unsigned int month, year;
-	unsigned long secs;
+	unsigned int month, year, secs;
 	int days;
 
 	/* time must be positive */
-	days = div_s64(time, 86400);
-	secs = time - (unsigned int) days * 86400;
+	days = div_s64_rem(time, 86400, &secs);
 
 	/* day of the week, 1970-01-01 was a Thursday */
 	tm->tm_wday = (days + 4) % 7;