Fix gdbserver regression exposed by gdb.threads/multi-create-ns-info-thr.exp

Commit 8629910955 ("Add thread_db_notice_clone to gdbserver")
introduced calls into libthread_db without making sure that the
current thread is pointing to a know-stopped thread.  This resulted in
sometimes thread_db_notice_clone failing->find_one_thread failing like
this, as seen when running gdb.threads/multi-create-ns-info-thr.exp:

~~~
  Thread <6> executing
  Thread <7> executing
  gdbserver: PID mismatch!  Expected 27472, got 27471
  gdbserver: Cannot find thread after clone.

  Thread <1000> executing
  Thread <1001> executing
~~~

Things go south from here and sometimes that ends up resulting in
gdbserver crashing and the test failing.

gdb/gdbserver/ChangeLog:
2017-09-29  Pedro Alves  <palves@redhat.com>

	* linux-low.c (handle_extended_wait): Pass parent thread instead
	of process to thread_db_notice_clone.
	* linux-low.h (thread_db_notice_clone): Replace parent process
	parameter with parent thread parameter.
	* thread-db.c (find_one_thread): Add comment.
	(thread_db_notice_clone): Replace parent process parameter with
	parent thread parameter.  Temporarily switch to the parent thread.

gdb/gdbserver/linux-low.c

@@ -656,7 +656,7 @@ handle_extended_wait (struct lwp_info **orig_event_lwp, int wstat)
 	  new_lwp->status_pending = status;
 	}
 
-      thread_db_notice_clone (get_thread_process (event_thr), ptid);
+      thread_db_notice_clone (event_thr, ptid);
 
       /* Don't report the event.  */
       return 1;