IB/hfi1: Fix memory leak during unexpected shutdown

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: Dean Luick <dean.luick@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>