Btrfs: check if there is enough space for balancing smarter

When checking if there is enough space for balancing a block group,
since we do not take raid types into consideration, we do not account
corrent amounts of space that we needed.  This makes us do some extra
work before we get ENOSPC.

Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>