This is an operational patch which may need little tweaks to add granularity for the vfs_op - right now write suspension et al will disable the feature. This can be easily dealt with by adding masks of what's prohibited. I'm posting this mostly to showcase additional uses for the feature added for pcpu counts.
With the patch in place contention on tmpfs locks when crossing the mount point is eliminated. On a system with root on zfs all contention is put on access checking.
zfs has an equivalent mechanism with rmlocks but it never re-populates the vnode. I'll patch that later. Naming is a little cumbersome but stolen from the same scheme for vnode ops - _lookup and _cachedlookup.