Approval request of some additions to src/sys/kern/vfs_subr.c and src/sys/sys/vnode.h

[Daichi GOTO]

Hi Konstantin :)

To fix a unionfs issue of http://www.freebsd.org/cgi/query-pr.cgi?pr=109377,
we need to add new functions

    void vkernrele(struct vnode *vp);
    void vkernref(struct vnode *vp);

and one value

    int	v_kernusecount; /* i ref count of kernel */

to src/sys/sys/vnode.h and rc/sys/kern/vfs_subr.c.

Unionfs will be panic when lower fs layer is forced umounted by
"umount -f".  So to avoid this issue, we've added
"v_kernusecount" value that means "a vnode count that kernel are
using".  vkernrele() and vkernref() are functions that manage
"v_kernusecount" value.

Please check those and give us an approve or some comments!

-- 
   Daichi GOTO, http://people.freebsd.org/~daichi