Ensure that dirent's d_off field is initialized

We have the d_off field in struct dirent for providing the seek offset
of the next directory entry. Several filesystems were not initializing
the field, which ends up being copied out to userland.

Reported by: Syed Faraz Abrar <faraz@elttam.com>
Reviewed by: kib

(cherry picked from commit 90f580b954090e669da234f6c8e8d0379ff9d8bc)
(cherry picked from commit acaac0eefa1f2675dfc49faa077e3fb10949d540)

Approved by: so
Security: CVE-2020-25578