Btrfs: fix inaccurate available space on raid0 profile

Btrfs: fix inaccurate available space on raid0 profile

When we use raid0 as the data profile, df command may show us a very
inaccurate value of the available space, which may be much less than the
real one. It may make the users puzzled. Fix it by changing the calculation
of the available space, and making it be more similar to a fake chunk
allocation.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
(cherry picked from commit 39fb26c398ddf8d7794a85e896cfe1a42e55524b)