rcu: Do not release a wait-head from a GP kthread

Fix a below race by not releasing a wait-head from the
GP-kthread as it can lead for reusing it whereas a worker
can still access it thus execute newly added callbacks too
early.

CPU 0                              CPU 1
-----                              -----

// wait_tail == HEAD1
rcu_sr_normal_gp_cleanup() {
    // has passed SR_MAX_USERS_WAKE_FROM_GP
    wait_tail->next = next;
    // done_tail = HEAD1
    smp_store_release(&rcu_state.srs_done_tail, wait_tail);
    queue_work() {
        test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work)
        __queue_work()
    }
}

                               set_work_pool_and_clear_pending()
                               rcu_sr_normal_gp_cleanup_work() {
// new GP, wait_tail == HEAD2
rcu_sr_normal_gp_cleanup() {
    // executes all completion, but stop at HEAD1
    wait_tail->next = HEAD1;
    // done_tail = HEAD2
    smp_store_release(&rcu_state.srs_done_tail, wait_tail);
    queue_work() {
        test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work)
        __queue_work()
    }
}
                                 // done = HEAD2
                                 done = smp_load_acquire(&rcu_state.srs_done_tail);
                                 // head = HEAD1
                                 head = done->next;
                                 done->next = NULL;
                                 llist_for_each_safe() {
                                 // completes all callbacks, release HEAD1
                                 }
                               }
                               // Process second queue
                               set_work_pool_and_clear_pending()
                               rcu_sr_normal_gp_cleanup_work() {
                               // done = HEAD2
                               done = smp_load_acquire(&rcu_state.srs_done_tail);

// new GP, wait_tail == HEAD3
rcu_sr_normal_gp_cleanup() {
    // Finds HEAD2 with ->next == NULL at the end
    rcu_sr_put_wait_head(HEAD2)
    ...

// A few more GPs later
rcu_sr_normal_gp_init() {
     HEAD2 = rcu_sr_get_wait_head();
     llist_add(HEAD2, &rcu_state.srs_next);
                               // head == rcu_state.srs_next
                               head = done->next;
                               done->next = NULL;
                               llist_for_each_safe() {
                                // EXECUTE CALLBACKS TOO EARLY!!!
                                }
                               }

Reported-by: Frederic Weisbecker <frederic@kernel.org>
Fixes: 05a10b921000 ("rcu: Support direct wake-up of synchronize_rcu() users")
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>