common/xfs: wipe the XFS superblock of each AGs

common/xfs: wipe the XFS superblock of each AGs

xfs/030 always fails after d0e484ac699f ("check: wipe scratch devices
between tests") get merged.

Due to xfs/030 does a sized(100m) mkfs. Before we merge above commit,
mkfs.xfs detects an old primary superblock, it will write zeroes to
all superblocks before formatting the new filesystem. But this won't
be done if we wipe the first superblock(by merging above commit).

That means if we make a (smaller) sized xfs after wipefs, those *old*
superblocks which created by last time mkfs.xfs will be left on disk.
Then when we do xfs_repair, if xfs_repair can't find the first SB, it
will go to find those *old* SB at first. When it finds them,
everyting goes wrong.

So I try to wipe each XFS superblock if there's a XFS ondisk, then
try to erase superblock of each XFS AG by default mkfs.xfs geometry.
Thanks Darrick J. Wong helped to analyze this issue.

Reported-by: Yang Xu <xuyang2018.jy@cn.fujitsu.com>
Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>