fat: provide option for setting timezone offset

So far FAT either offsets time stamps by sys_tz.minuteswest or leaves them
as they are (when tz=UTC mount option is used).  However in some cases it
is useful if one can specify time stamp offset on his own (e.g.  when time
zone of the camera connected is different from time zone of the computer,
or when HW clock is in UTC and thus sys_tz.minuteswest == 0).

So provide a mount option time_offset= which allows user to specify offset
in minutes that should be applied to time stamps on the filesystem.

akpm: this code would work incorrectly when used via `mount -o remount',
because cached inodes would not be updated.  But fatfs's fat_remount() is
basically a no-op anyway.

Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fat/misc.c

@@ -212,8 +212,10 @@ void fat_time_fat2unix(struct msdos_sb_info *sbi, struct timespec *ts,
 		   + days_in_year[month] + day
 		   + DAYS_DELTA) * SECS_PER_DAY;
 
-	if (!sbi->options.tz_utc)
+	if (!sbi->options.tz_set)
 		second += sys_tz.tz_minuteswest * SECS_PER_MIN;
+	else
+		second -= sbi->options.time_offset * SECS_PER_MIN;
 
 	if (time_cs) {
 		ts->tv_sec = second + (time_cs / 100);
@@ -229,8 +231,9 @@ void fat_time_unix2fat(struct msdos_sb_info *sbi, struct timespec *ts,
 		       __le16 *time, __le16 *date, u8 *time_cs)
 {
 	struct tm tm;
-	time_to_tm(ts->tv_sec, sbi->options.tz_utc ? 0 :
-		   -sys_tz.tz_minuteswest * 60, &tm);
+	time_to_tm(ts->tv_sec,
+		   (sbi->options.tz_set ? sbi->options.time_offset :
+		   -sys_tz.tz_minuteswest) * SECS_PER_MIN, &tm);
 
 	/*  FAT can only support year between 1980 to 2107 */
 	if (tm.tm_year < 1980 - 1900) {