xfs: fix iget usage in directory scrub

These two patches fix some problems in the directory scrubbing code with
regards to how and when we check the children of a directory.  The first
adds UNTRUSTED to the child iget call so that we can detect a dirent
that points to an unallocated inode.  The second fixes a bug where we
weren't checking the inode pointed to by dotdot entries at all.
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