xfs_repair: enforce that inode btree chunks can't point to AG headers

xfs_repair: enforce that inode btree chunks can't point to AG headers

xfs_repair has a very old check that evidently excuses the AG 0 inode
btrees pointing to blocks that are already marked XR_E_INUSE_FS* (e.g.
AG headers).  mkfs never formats filesystems that way and it looks like
an error, so purge the check.  After this, we always complain if inodes
overlap with AG headers because that should never happen.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
[sandeen: removed unused switch cases that fall through to default]
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>