btrfs: volumes: Cleanup stripe size calculation

Cleanup the following things:
1) open coded SZ_16M round up
2) use min() to replace open-coded size comparison
3) code style

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Gu Jinxiang <gujx@cn.fujitsu.com>
[ reformat comment ]
Signed-off-by: David Sterba <dsterba@suse.com>