CFR: NFS server vnode locking patch

I managed to find some time to take a closer look at vnode locking in
the NFS server code and found that the situation was worse than I
initially thought.  I've put together a patch that seems to fix all the
bugs that I found.  With this patch, the code passes the simple tests
that I wrote as well as NFS mounting a local directory on /usr/obj and
running "make -j10 buildworld" (after I cranked up vfs.hirunningspace
and vfs.lorunningspace by 50x to avoid the wdrain bio deadlock I
mentioned yesterday), all with the DEBUG_VFS_LOCKS kernel option
enabled.

The NFS server code was in bad shape from being hacked on too many times
before I touched it and it looks like it has accumulated some historical
baggage, and my changes certainly don't help. I attempted to match the
existing style and control flow since I wanted to minimize the changes
at the time to avoid introducing new bugs, but this meant that I had to
duplicate some code in a number of places.

I saw two possible ways of getting the initial dirp attributes.  One was
to set LOCKPARENT on the first lookup() call in nfs_namei() and cap
VOP_GETATTR() at that point.  I chose the other possible implementation,
which was to temporarily lock the dirp and call VOP_GETATTR() before the
loop, because this change was simpler.

The NFS server code badly needs a rewrite by someone who understands it
well.

I'm hoping to get enough review and testing of this patch so that I can
get re approval to fix vnode locking in the NFS server code for
5.1.