pids: Compute task_tgid using signal->leader_pid

The cost is the the same and this removes the need
to worry about complications that come from de_thread
and group_leader changing.

__task_pid_nr_ns has been updated to take advantage of this change.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

include/net/scm.h

@@ -8,6 +8,7 @@
 #include <linux/security.h>
 #include <linux/pid.h>
 #include <linux/nsproxy.h>
+#include <linux/sched/signal.h>
 
 /* Well, we should have at least one descriptor open
  * to accept passed FDs 8)