kernel - Fix various memory & swap leaks in tmpfs

kernel - Fix various memory & swap leaks in tmpfs

* tmpfs was using a vref() instead of a vget() (cache_vget() to be precise)
  which does not necessarily reactivate an inactive vnode.  This can cause
  the vnode to languish in the vnode cache even when the underlying file
  has been removed and no longer has any open() descriptors.

* tmpfs_nrmdir(), tmpfs_nremove(), and tmpfs_nrename() (for target replacement
  renames) was effected.

Reported-by: sephe and others, tested by sephe, dillon