linux(4): Fixup miscalculation of d_off of struct dirent in getdents() syscalls.

Avoid calculating d_off value as it is specific to the underlying filesystem
and can be used by others API, like lseek(), seekdir() as input offset.

Differential revision: https://reviews.freebsd.org/D31551
MFC after: 2 weeks

(cherry picked from commit 099fa2feb36fe6a68a87dfdb0f290b882139a88d)