btrfs: use PAGE_ALIGNED instead of IS_ALIGNED

The <linux/mm.h> already provides the PAGE_ALIGNED macro. Let's
use it instead of IS_ALIGNED and passing PAGE_SIZE directly.

Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Fanjun Kong <bh1scw@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>