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
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27792