Implement checking of .' and ..' entries of subdirectory.

Reviewed by: pfg
Obtained from: Android https://android.googlesource.com/platform/external/fsck_msdos/+/b6ee08aadb580341a4d80943741b80de16a88b5d%5E%21/
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D19824