Use vnsz2log directly in assertion on its relation to sizeof(struct vnode).

This reduces the size of diffs required to support different values of
vnsz2log. In CheriBSD, kernels for CHERI architectures have vnodes
larger than 512 bytes and require a value of 9.

Reviewed by: mjg
Obtained from: CheriBSD
Sponsored by: University of Cambridge, Google, Inc.
Differential Revision: https://reviews.freebsd.org/D34418