Ensure that d_off is initialized

[markj]

Summary

We have the d_off field in struct dirent for providing the offset of the
next directory entry. Several filesystems did not populate the field,
which ends up being leaked.

Reported by: Syed Faraz Abrar <[e-mail]>

[trasz]

I don't remember much about how directory entries work, but I don't see anything wrong with the patch. I have a question, though: how did it work before? Is that field actually used?

[kib]

d_off was added during ino64 work, for supposed benefit of nfs server. I believe the idea was that d_off use could replace cookies.

Right now it is not used, and d_off presence is optional. If d_off is zero, userspace has to use other methods (lseek ?) to bookmark directory position.