This revision began as a simple change to eliminate an uninitialized warning
found by Coverity. However, upon closer inspection the implementation of
fsck_ffs's fsck_readdir() and dircheck() functions is both nearly impossible
to follow and fails to check / fix directories in several cases. So, this
revision is an entire rewrite of these two functions to clarify what they
are doing and also to get something that works properly.

Referred by: cem
Reviewed by: kib, David G Lawrence
MFC after: 3 days
CID 1401317: namlen may be used uninitialized