modifying nullfs

[Matthew Dillon]

    Most nullfs VOP's are going to go directly to the underlying
    filesystem and NOT run through nullfs itself.

    In DragonFly we don't have to replicate the vnode infrastructure for
    directory nodes in nullfs because we track { mp, vnode } instead of
    just { vnode }.  nullfs is basically only used to track the
    mount structure.  Our namecache code handles mount points via the
    chaining within the mount structures and NOT via chaining within
    directory vnodes.

    In otherwords, in DragonFly a nullfs mount as just as good as the
    underlying filesystem mount, with no added overhead to use it.

					-Matt
					Matthew Dillon 
					<dillon at backplane.com>