xfs: don't accept inode buffers with suspicious unlinked chains

When we're verifying inode buffers, sanity-check the unlinked pointer.
We don't want to run the risk of trying to purge something that's
obviously broken.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>