Do not copy vp into f_data for DTYPE_VNODE files.

The pointer to vnode is already stored into f_vnode, so f_data can be
reused. Fix all found users of f_data for DTYPE_VNODE.

Provide finit_vnode() helper to initialize file of DTYPE_VNODE type.

Reviewed by: markj (previous version)
Discussed with: freqlabs (openzfs chunk)
Tested by: pho (previous version)
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D26346