vfs: eliminate v_tag from struct vnode

[mjg]

Summary

The only consumer uses it in a bogus manner, replace the use with an equivalent bogosity which does not require the field.

This also means getnewvnode takes the argument unnecessarily. I'll make a sweep to remove it later.

[kib]

head/sys/security/mac_veriexec/mac_veriexec.c

754 ↗

(On Diff #66431)

What you can do is (struct vnode *)(vp->v_object->handle) for object != NULL && object->type == OBJT_VNODE. The only case that would be not handled by this expression is nullfs mount over tmpfs.