Eliminate an unnecessary rerun request in fsck_ffs.

When fsck_ffs is running in preen mode and finds a zero-length directory,
it deletes that directory. In doing this operation, it unnecessary set
its internal flag saying that fsck_ffs needed to be rerun. This patch
deletes the rerun request for this case.

Reported by: Mark Johnson
PR: 246962
MFC after: 1 week
Sponsored by: Netflix