Btrfs: don't pre-allocate btrfs bio

We pre-allocate a btrfs bio with fixed size, and then may re-allocate
memory if we find stripes are bigger than the fixed size. But this
pre-allocation is not necessary.

Also we don't have to calcuate the stripe number twice.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>