xfs_scrub: don't revisit scanned inodes when reprocessing a stale inode

xfs_scrub: don't revisit scanned inodes when reprocessing a stale inode

If we decide to restart an inode chunk walk because one of the inodes is
stale, make sure that we don't walk an inode that's been scanned before.
This ensure we always make forward progress.

Found by observing backwards inode scan progress while running xfs/285.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
[sandeen: add comment above forward-progress test]
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>