Btrfs: fix easily get into ENOSPC in mixed case

When a btrfs disk is created by mixed data & metadata option, it will have no
pure data or pure metadata space info.

In btrfs's for-linus branch, commit 78b1ea13838039cd88afdd62519b40b344d6c920
(Btrfs: fix OOPS of empty filesystem after balance) initializes space infos at
the very beginning.  The problem is this initialization does not take the mixed
case into account, which will cause btrfs will easily get into ENOSPC in mixed
case.

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