tmpfs - Fix races in tmpfs_nrename() and tmpfs_nrmdir()

tmpfs - Fix races in tmpfs_nrename() and tmpfs_nrmdir()

* Lock all nrename elements before checks.  This is particularly
  important when renaming over a file or empty directory, but other
  manipulations done by this code without locks could also cause
  races which result in corruption, particularly with the link count.

* Lock all nrmdir elements before checks, for the same reason.