jbd2: fix hung processes in jbd2_journal_lock_updates()

Toshiyuki Okajima found out that when running

for ((i=0; i < 100000; i++)); do
        if ((i%2 == 0)); then
                chattr +j /mnt/file
        else
                chattr -j /mnt/file
        fi
        echo "0" >> /mnt/file
done

process sometimes hangs indefinitely in jbd2_journal_lock_updates().

Toshiyuki identified that the following race happens:

jbd2_journal_lock_updates()            |jbd2_journal_stop()
---------------------------------------+---------------------------------------
 write_lock(&journal->j_state_lock)    |    .
 ++journal->j_barrier_count            |    .
 spin_lock(&tran->t_handle_lock)       |    .
 atomic_read(&tran->t_updates) //not 0 |
                                       | atomic_dec_and_test(&tran->t_updates)
                                       |    // t_updates = 0
                                       | wake_up(&journal->j_wait_updates)
 prepare_to_wait()                     |    // no process is woken up.
 spin_unlock(&tran->t_handle_lock)     |
 write_unlock(&journal->j_state_lock)  |
 schedule() // never return            |

We fix the problem by first calling prepare_to_wait() and only after that
checking t_updates in jbd2_journal_lock_updates().

Reported-and-analyzed-by: Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>