vfs: store precomputed namecache hash in the vnode

This significantly speeds up path lookup, Cascade Lake doing access(2) on ufs
on /usr/obj/usr/src/amd64.amd64/sys/GENERIC/vnode_if.c, ops/s:
before: 2535298
after: 2797621

Over +10%.

The reversed order of computation here does not seem to matter for hash
distribution.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D25921