Call vget(9) in uvn_put() to make sure we hold a reference if we're called

Call vget(9) in uvn_put() to make sure we hold a reference if we're called
by the pagedaemon for a persisting vnode.  This prevents a panic in
uvm_vnp_unache() since a persisting vnode has a reference count of zero.
This should also prevent the vnode from being recycled from under our
feet while we're doing I/O to it.

This fixes the "macppc panic: vref used where vget required" issue.

tested by bluhm@, tb@
ok mpi@