This MFC includes changes to better manage the vnode freelist
and to streamline the allocation and freeing of vnodes.

Note that to maintain the KPI the VI_AGE flag is left defined
in sys/vnode.h though its use is dropped as described in 291380.
To maintain KBI the vfs.vlru_alloc_cache_src sysctl variable
remains though it no longer has any effect as described in 291244.

Move the comment about resident pages preventing vnode from leaving
active list, into the header comment for vdrop(), which is the
function that decides whether to leave the vnode on the list.  Note
that dirty page write-out in vinactive() is asynchronous.

Discussed with: alc
Sponsored by:   The FreeBSD Foundation

MFC of 291380:
Remove VI_AGE vnode iflag, it is unused.

Noted by:       bde
Sponsored by:   The FreeBSD Foundation

For performance reasons, it is useful to have a single string used as
the name of a filesystem when setting it as the first parameter to the
getnewvnode() function. Most filesystems call getnewvnode from just one
place so can use a literal string as the first parameter. However, NFS
calls getnewvnode from two places, so we create a global constant string
that can be used by the two instances. This change also collapses two
instances of getnewvnode() in the UFS filesystem to a single call.

Reviewed by: kib
Tested by:   Peter Holm

As the kernel allocates and frees vnodes, it fully initializes them
on every allocation and fully releases them on every free.  These
are not trivial costs: it starts by zeroing a large structure then
initializes a mutex, a lock manager lock, an rw lock, four lists,
and six pointers. And looking at vfs.vnodes_created, these operations
are being done millions of times an hour on a busy machine.

As a performance optimization, this code update uses the uma_init
and uma_fini routines to do these initializations and cleanups only
as the vnodes enter and leave the vnode_zone. With this change the
initializations are only done kern.maxvnodes times at system startup
and then only rarely again. The frees are done only if the vnode_zone
shrinks which never happens in practice. For those curious about the
avoided work, look at the vnode_init() and vnode_fini() functions in
kern/vfs_subr.c to see the code that has been removed from the main
vnode allocation/free path.

Reviewed by: kib
Tested by:   Peter Holm

We need to zero out the union of pointers in a freed vnode structure.

Fix from:  Mateusz Guzik
Tested by: Jason Unovitch

We need to zero out the clustering variables in a freed vnode structure.
For completeness add a VNASSERT that there are no threads waiting on a
range lock (this was previously checked on every vnode free).

Reported by; Rick Macklem
Fix from:    Mateusz Guzik