audit: Initialize vattr fields before calling VOP_GETATTR

Some filesystems do not fill out certain optional vattr fields. To
ensure that they do not get copied out to userspace uninitialized, use
VATTR_NULL to provide default values.

Reported by: KMSAN
Sponsored by: The FreeBSD Foundation

(cherry picked from commit ecd764b0ea7a1b7fb5f1e6632f8d8db9233f06ea)