devel/libgtop: Fix build on 14-CURRENT post 2bfd8992c7c7

sys/inode.h now implicitly includes sys/buf.h which which itself has
dependencies requiring repositioning of inode.h.

This patch also references the inode itself instead of relying on the VTOI
macro thereby removing the need to define the _KERNEL macro when including
sys/inode.h.

PR: 253928, 253870
Submitted by: nyan _ myuji.xyz
Reported by: cy, madpilot
Reviewed by: cy
MFH: 2021Q1
Differential Revision: https://reviews.freebsd.org/D29050