Correct a bug in seekdir/readdir which could cause the directory entry

Correct a bug in seekdir/readdir which could cause the directory entry
after a deleted entry to be skipped when seeking past the deleted entry.

NOTE: DragonFly has a specific issue even after this fix which currently
causes seekdirs to be unreliable if any files are deleted.  DragonFly
translates directory entries into a filesystem-independant form and if
the real filesystem collapses the entry, the offsets will not be maintained
in the machine-independant form.

Submitted-by: Marc Balmer <marc@msys.ch>