erofs: Validate directory '.'/'..' entries and nlink counts

When going over dumpfile canonicalization related to hardlinks,
it made me think about the potential for skew between EROFS
nlink on inode vs what's actually present.

This then led to: our reader should be enforcing this matches.
And while there, my agent also pointed out we could be checking `.`/`..`
among other things.

The previously unused InvalidSelfReference and InvalidParentReference
error variants are now wired up.

Assisted-by: OpenCode (Claude Opus 4)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters