btrfs: Use IS_ALIGNED in btrfs_truncate_block instead of opencoding it

No functional changes, just makes the code more readable

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>