During an unexpected shutdown, references to tid_rb_node were NULL'ed out
without properly being released.

Fix this by calling clear_tid_node in the mmu notifier remove callback
rather than after these callbacks are called.
Reviewed-by: NDean Luick <dean.luick@intel.com>
Signed-off-by: NIra Weiny <ira.weiny@intel.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>