tmpfs - Close rare vnode recycle race

tmpfs - Close rare vnode recycle race

* Keep the node lock held when clearing tn_vnode in tmpfs_reclaim()
  to protect against a use-after-free race on tn_vnode against another
  thread.

* Keep the node locked across the node type check and vnode
  ref in tmpfs_unmount() to protect against asynchronous
  reclamation races.