xfs: fix errors in attr/directory scrubbers

During a code audit of the effectiveness of the online repair code,
several deficiencies were discovered in the extended attribute and
directory scrubbing code.  The first two patches change both scrubbers
to note corruption when the name-based lookups (which test the hash
tree) fail to return any information.  The final patch amends the
directory checker to note corruption the inode target of a directory
entry points to an unallocated inode.

In v2 we declutter the error handling a bit.