btrfs: fix percent calculation for bg reclaim message

We have a report, that the info message for block-group reclaim is
crossing the 100% used mark.

This is happening as we were truncating the divisor for the division
(the block_group->length) to a 32bit value.

Fix this by using div64_u64() to not truncate the divisor.

In the worst case, it can lead to a div by zero error and should be
possible to trigger on 4 disks RAID0, and each device is large enough:

  $ mkfs.btrfs  -f /dev/test/scratch[1234] -m raid1 -d raid0
  btrfs-progs v6.1
  [...]
  Filesystem size:    40.00GiB
  Block group profiles:
    Data:             RAID0             4.00GiB <<<
    Metadata:         RAID1           256.00MiB
    System:           RAID1             8.00MiB

Reported-by: Forza <[email protected]>
Link: https://lore.kernel.org/linux-btrfs/[email protected]/
Fixes: 5f93e77 ("btrfs: zoned: print unusable percentage when reclaiming block groups")
CC: [email protected] # 5.15+
Reviewed-by: Anand Jain <[email protected]>
Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Johannes Thumshirn <[email protected]>
Reviewed-by: David Sterba <[email protected]>
[ add Qu's note ]
Signed-off-by: David Sterba <[email protected]>

Author: morbidrsa

fs/btrfs/block-group.c

@@ -1826,7 +1826,8 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
 
 		btrfs_info(fs_info,
 			"reclaiming chunk %llu with %llu%% used %llu%% unusable",
-				bg->start, div_u64(bg->used * 100, bg->length),
+				bg->start,
+				div64_u64(bg->used * 100, bg->length),
 				div64_u64(zone_unusable * 100, bg->length));
 		trace_btrfs_reclaim_block_group(bg);
 		ret = btrfs_relocate_chunk(fs_info, bg->start);