kernel/sys.c: fix missing rcu protection for sys_getpriority()

find_task_by_vpid() is not safe without rcu_read_lock().  2.6.33-rc7 got
RCU protection for sys_setpriority() but missed it for sys_getpriority().
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/sys.c

@@ -222,6 +222,7 @@ SYSCALL_DEFINE2(getpriority, int, which, int, who)
 	if (which > PRIO_USER || which < PRIO_PROCESS)
 		return -EINVAL;
 
+	rcu_read_lock();
 	read_lock(&tasklist_lock);
 	switch (which) {
 		case PRIO_PROCESS:
@@ -267,6 +268,7 @@ SYSCALL_DEFINE2(getpriority, int, which, int, who)
 	}
 out_unlock:
 	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 
 	return retval;
 }