btrfs: Use explicit round_down macro in btrfs resize ioctl handler

No functional changes, just make the code more self-explanatory.
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/ioctl.c

@@ -1569,8 +1569,7 @@ static noinline int btrfs_ioctl_resize(struct file *file,
 		goto out_free;
 	}
 
-	new_size = div_u64(new_size, fs_info->sectorsize);
-	new_size *= fs_info->sectorsize;
+	new_size = round_down(new_size, fs_info->sectorsize);
 
 	btrfs_info_in_rcu(fs_info, "new size for %s is %llu",
 			  rcu_str_deref(device->name), new_size);