Try to decrease the number of bugs in unionfs after the VV_TEXT flag removal.

• Provide unionfs_add_writecount() which passes the writecount to the lower or upper vnode as appropriate.
• In unionfs VOP_RECLAIM() implementation, annulate unionfs writecounts from upper or lower vnode. It is not clear that it is always correct to remove the all references from either lower or upper vnode, but we currently do not track which vnode get how many refs anyway.

Reported and tested by: t_uemura@macome.co.jp
MFC after: 1 week
Sponsored by: The FreeBSD Foundation