MFC: r297837
Bruce Evans reported that there was a performance regression between
the old and new NFS clients. He did a good job of isolating the problem
which was caused by the new NFS client not setting the post write mtime
correctly. The new NFS client code was cloned from the old client, but
was incorrect, because the mtime in the nfs vnode's cache wasn't yet
updated. This patch fixes this problem. The patch also adds missing mutex
locking.