xfs: avoid f_bfree overflow

hulk inclusion
category: bugfix
bugzilla: 31088
CVE: NA

---------------------------

If fdblocks < mp->m_alloc_set_aside, statp->f_bfree will overflow.
When we df -h /mnt(xfs mount point), will show this:
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop0       17M  -64Z  -32K 100% /mnt

We can construct an img like this:

dd if=/dev/zero of=xfs.img bs=1M count=20
mkfs.xfs -d agcount=1 xfs.img
xfs_db -x xfs.img
sb 0
write fdblocks 0
agf 0
write freeblks 0
write longest 0
quit

Make sure statp->f_bfree does not underflow.

Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>