f2fs: fix incorrect bimodal calculation

In update_sit_info, we use div_u64 to handle 'u64 divide u64' case, but
div_u64 can only handle 32-bits divisor, so our divisor with u64 type
passed to div_u64 will overflow, result in the wrong calculation when
show debug info of f2fs as below:

BDF: 464, avg. vblocks: 23509
(BDF should never exceed 100)

So change to use div64_u64 to handle this case correctly.
Signed-off-by: NChao Yu <chao2.yu@samsung.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/debug.c

@@ -118,7 +118,7 @@ static void update_sit_info(struct f2fs_sb_info *sbi)
 		}
 	}
 	dist = div_u64(MAIN_SECS(sbi) * hblks_per_sec * hblks_per_sec, 100);
-	si->bimodal = div_u64(bimodal, dist);
+	si->bimodal = div64_u64(bimodal, dist);
 	if (si->dirty_count)
 		si->avg_vblocks = div_u64(total_vblocks, ndirty);
 	else