BACKPORT: FROMLIST: pids: make task_tgid_nr_ns() safe

This was reported many times, and this was even mentioned in commit
52ee2dfd "pids: refactor vnr/nr_ns helpers to make them safe" but
somehow nobody bothered to fix the obvious problem: task_tgid_nr_ns()
is not safe because task->group_leader points to nowhere after the
exiting task passes exit_notify(), rcu_read_lock() can not help.

We really need to change __unhash_process() to nullify group_leader,
parent, and real_parent, but this needs some cleanups. Until then we
can turn task_tgid_nr_ns() into another user of __task_pid_nr_ns() and
fix the problem.

Reported-by: Troy Kensinger <tkensinger@google.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

(url: https://patchwork.kernel.org/patch/9913055/)
Bug: 31495866

Change-Id: I5e67b02a77e805f71fa3a787249f13c1310f02e2
(cherry picked from commit 48de73e34a5760591758ff7ffdc79bc464e60f69)