btrfs: round down size diff when shrinking/growing device

Further testing showed that the fix introduced in 7dfb8be ("btrfs:
Round down values which are written for total_bytes_size") was
insufficient and it could still lead to discrepancies between the
total_bytes in the super block and the device total bytes. So this patch
also ensures that the difference between old/new sizes when
shrinking/growing is also rounded down. This ensure that we won't be
subtracting/adding a non-sectorsize multiples to the superblock/device
total sizees.

Fixes: 7dfb8be ("btrfs: Round down values which are written for total_bytes_size")
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -2702,7 +2702,7 @@ int btrfs_grow_device(struct btrfs_trans_handle *trans,
 
 	mutex_lock(&fs_info->chunk_mutex);
 	old_total = btrfs_super_total_bytes(super_copy);
-	diff = new_size - device->total_bytes;
+	diff = round_down(new_size - device->total_bytes, fs_info->sectorsize);
 
 	if (new_size <= device->total_bytes ||
 	    device->is_tgtdev_for_dev_replace) {
@@ -4406,7 +4406,7 @@ int btrfs_shrink_device(struct btrfs_device *device, u64 new_size)
 	u64 diff;
 
 	new_size = round_down(new_size, fs_info->sectorsize);
-	diff = old_size - new_size;
+	diff = round_down(old_size - new_size, fs_info->sectorsize);
 
 	if (device->is_tgtdev_for_dev_replace)
 		return -EINVAL;