Btrfs: Simplify num_stripes's calculation logical for __btrfs_alloc_chunk()

We can use this simple method to make source more readable.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>