Allow shared-locked vnode to be passed to vunref(9). When shared-locked vnode is supplied as an argument to vunref(9) and resulting usecount is 0, set VI_OWEINACT and do not try to upgrade vnode lock. The later could cause vnode unlock, allowing the vnode to be reclaimed meantime.
Allow shared-locked vnode to be passed to vunref(9).
When shared-locked vnode is supplied as an argument to vunref(9) and
resulting usecount is 0, set VI_OWEINACT and do not try to upgrade vnode
lock. The later could cause vnode unlock, allowing the vnode to be
reclaimed meantime.

Tested by:	pho
MFC after:	1 week

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