sys_getsid: don't use ->nsproxy directly

With the new semantics of find_vpid() we don't need to play with ->nsproxy
explicitely, _vxx() do the right things.

Also s/tasklist/rcu/.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/sys.c

@@ -1025,19 +1025,16 @@ asmlinkage long sys_getsid(pid_t pid)
 	else {
 		int retval;
 		struct task_struct *p;
-		struct pid_namespace *ns;
-
-		ns = current->nsproxy->pid_ns;
 
-		read_lock(&tasklist_lock);
-		p = find_task_by_pid_ns(pid, ns);
+		rcu_read_lock();
+		p = find_task_by_vpid(pid);
 		retval = -ESRCH;
 		if (p) {
 			retval = security_task_getsid(p);
 			if (!retval)
-				retval = task_session_nr_ns(p, ns);
+				retval = task_session_vnr(p);
 		}
-		read_unlock(&tasklist_lock);
+		rcu_read_unlock();
 		return retval;
 	}
 }