remove the no longer needed pid_alive() check in __task_pid_nr_ns()

Starting from 2c4704756cab ("pids: Move the pgrp and session pid pointers
from task_struct to signal_struct") __task_pid_nr_ns() doesn't dereference
task->group_leader, we can remove the pid_alive() check.

pid_nr_ns() has to check pid != NULL anyway, pid_alive() just adds the
unnecessary confusion.

Signed-off-by: Oleg Nesterov <[e-mail]>
Reviewed-by: "Eric W. Biederman" <[e-mail]>
Acked-by: Christian Brauner <[e-mail]>
Signed-off-by: Eric W. Biederman <[e-mail]>

kernel/pid.c

···
 	rcu_read_lock();
 	if (!ns)
 		ns = task_active_pid_ns(current);
-	if (likely(pid_alive(task)))
-		nr = pid_nr_ns(rcu_dereference(*task_pid_ptr(task, type)), ns);
+	nr = pid_nr_ns(rcu_dereference(*task_pid_ptr(task, type)), ns);
 	rcu_read_unlock();
 
 	return nr;