ioprio: rcu protect find_task_by_vpid call

Since commit 4221a9918e38b7494cee341dda7b7b4bb8c04bde
find_task_by_pid_ns call needs to be protected with RCU lock.

Tetsuo Handa wrote:

| Usually tasklist gives enough protection, but if copy_process() fails

| it calls free_pid() lockless and does call_rcu(delayed_put_pid().

| This means, without rcu lock find_pid_ns() can't scan the hash table

| safely.

"Unsafe" find_task_by_pid_ns call may look like this:

Call Trace:

 [<ffffffff810656f2>] lockdep_rcu_dereference+0xaa/0xb2
 [<ffffffff81053c67>] find_task_by_pid_ns+0x4f/0x68
 [<ffffffff81053c9d>] find_task_by_vpid+0x1d/0x1f
 [<ffffffff811104e2>] sys_ioprio_get+0x50/0x2da
 [<ffffffff81002182>] system_call_fastpath+0x16/0x1b

V2: rcu critical section expanded according to comment
by Paul E. McKenney.

The patch below adds missing rcu in sys_ioprio_{set|get}.

--- a/fs/ioprio.c
+++ b/fs/ioprio.c
@@ -111,12 +111,14 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio)
        read_lock(&tasklist_lock);
        switch (which) {
                case IOPRIO_WHO_PROCESS:
+                       rcu_read_lock();
                        if (!who)
                                p = current;
                        else
                                p = find_task_by_vpid(who);
                        if (p)
                                ret = set_task_ioprio(p, ioprio);
+                       rcu_read_unlock();
                        break;
                case IOPRIO_WHO_PGRP:
                        if (!who)
@@ -205,12 +207,14 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
        read_lock(&tasklist_lock);
        switch (which) {
                case IOPRIO_WHO_PROCESS:
+                       rcu_read_lock();
                        if (!who)
                                p = current;
                        else
                                p = find_task_by_vpid(who);
                        if (p)
                                ret = get_task_ioprio(p);
+                       rcu_read_unlock();
                        break;
                case IOPRIO_WHO_PGRP:
                        if (!who)