xfstests: an regression test for btrfs balance

xfstests: an regression test for btrfs balance

Originally, when executing "btrfs balance" right after
"btrfs subvolume snaphot" & "btrfs subvolume delete",
a kernel BUG arises.

This problem is caused by the patch:
[PATCH 1/2] Btrfs: fix for patch "cleanup: don't check
the same thing twice"
The commit id: 48475471728f060bfd2e686f592ef208d3ba8b7d
(in kernel/git/torvalds/linux.git)

handled by the patch:
[PATCH 2/3] Btrfs: fix oops caused by the space balance
and dead roots

[rjohnston: change test number to 14]

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Reviewed-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>