f2fs: fix to update mtime correctly

If we change system time to the past, get_mtime() will return a
overflowed time, and SIT_I(sbi)->max_mtime will be udpated
incorrectly, this patch fixes the two issues.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
index ca41798..0f8a6b9 100644
--- a/fs/f2fs/checkpoint.c
+++ b/fs/f2fs/checkpoint.c
@@ -1233,7 +1233,7 @@
 	 * modify checkpoint
 	 * version number is already updated
 	 */
-	ckpt->elapsed_time = cpu_to_le64(get_mtime(sbi));
+	ckpt->elapsed_time = cpu_to_le64(get_mtime(sbi, true));
 	ckpt->free_segment_count = cpu_to_le32(free_segments(sbi));
 	for (i = 0; i < NR_CURSEG_NODE_TYPE; i++) {
 		ckpt->cur_node_segno[i] =