Fix a HAMMER assertion which turned out to be a bug in VOP_N*(). Sometimes

Fix a HAMMER assertion which turned out to be a bug in VOP_N*().  Sometimes
the dvp passed to these functions can be reclaimed.  The locked leaf
namecache node is not sufficient to prevent its parent directory from
being reclaimed under heavy loads.

Instead of trying to play cute tricks, actually do a formal reference of
the dvp.  We don't have to lock it, though.