Freeing vnodes.

[Jeff Roberson]

I have a patch at http://www.chesapeake.net/~jroberson/freevnodes.diff
that allows us to start reclaiming vnodes from the free list and release
their memory.  It also changes the semantics of wantfreevnodes, and makes
getnewvnode() much prettier.

The changes attempt to keep some number of vnodes, currently 2.5% of
desiredvnodes, that are free in memory.  Free vnodes are vnodes which
have no references or pages in memory.  For example, if an application
simply stat's a vnode, it will end up on the free list at the end of the
operation.  The algorithm that is currently in place will immediately
recycle these vnodes once there is enough pressure, which will cause us to
do a full lookup and reread the inode, etc. as soon as it is stat'd again.

This also removes the recycling from the getnewvnode() path.  Instead, it
is done by a new helper function that is called from vnlru_proc().  This
function just frees vnodes from the head of the list until we reach our
wantfreevnodes target.

I haven't perf tested this yet, but I have a box that is doing a
buildworld with a fairly constant freevnodes count which shows that vnodes
are actually being uma_zfree'd.

Comments?  Anyone willing to do some perf tests for me?

Thanks,
Jeff