Fix a race between the namecache and the vnode recycler. A vnode cannot be

Fix a race between the namecache and the vnode recycler.  A vnode cannot be
recycled if it's namecache entry represents a directory with locked children.
The various VOP_N*() functions require the parent dvp to be stable.

The main fix is in vrecycle() (kern/vfs_subr.c).  Do not vgone() the vnode
if we can't clean out the children.

Also create an API to assert that the parent dvp is stable, and make it
vhold/vdrop the dvp.

The race primarily effected HAMMER which uses the VOP_N*() API.