kern/vfs: Add the per-filesystem vnode counter to struct mount.

[seigo.tanimura_gmail.com]

Summary

This is the preparation for the precise vnode recycle on ZFS and nullfs.
In the upcoming commits, those filesystems make mnt_fsvninusep point to the
vnode counter internal in each filesystem.

This change does not affect any filesystems that do not set mnt_fsvninusep.

This change is separated from the ZFS and nullfs fixes because it modifies
struct mount.

PR: 275594
Security: FreeBSD-EN-23:18.openzfs
Signed-off-by: Seigo Tanimura <[e-mail]>

[seigo.tanimura_gmail.com]

Target branches: main, stable/14, releng/14.0