can sleep on the vnode lock, while vget is sleeping on the
VI_INACTNOW flag (or the vget caller is looping on vget returning failure
because of the VI_INACTNOW flag). With layered FSes, the upper and lower
vnodes share the same lock, so the vget() caller above can be already
holding the vnode lock.
Fix by dropping VI_INACTNOW before sleeping on the vnode lock in
vrelel(), and check the ref count again once we have the lock. If the
vnode has more than one reference, donc VOP_INACTIVE it.
Fix PR kern/42318 and PR kern/42377
patch tested by Hisashi T Fujinaka, Joachim König, Stephen Borrill and
Matthias Scheler.
8c392da154