qa/tasks/rbd_fio: bump default fio version to 2.21

qa/tasks/rbd_fio: bump default fio version to 2.21

I'm seeing sporadic single thread deadlocks on fio stat_mutex during krbd
thrash runs:

  (gdb) info threads
    Id   Target Id         Frame
  * 1    Thread 0x7f89ee730740 (LWP 15604) 0x00007f89ed9f41bd in __lll_lock_wait () from /lib64/libpthread.so.0
  (gdb) bt
  #0  0x00007f89ed9f41bd in __lll_lock_wait () from /lib64/libpthread.so.0
  #1  0x00007f89ed9f17b2 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  #2  0x00000000004429b9 in fio_mutex_down (mutex=0x7f89ee72d000) at mutex.c:170
  #3  0x0000000000459704 in thread_main (data=<optimized out>) at backend.c:1639
  #4  0x000000000045b013 in fork_main (offset=0, shmid=<optimized out>, sk_out=0x0) at backend.c:1778
  #5  run_threads (sk_out=sk_out@entry=0x0) at backend.c:2195
  #6  0x000000000045b47f in fio_backend (sk_out=sk_out@entry=0x0) at backend.c:2400
  #7  0x000000000040cb0c in main (argc=2, argv=0x7fffad3e3888, envp=<optimized out>) at fio.c:63
  (gdb) up 2
  170                     pthread_cond_wait(&mutex->cond, &mutex->lock);
  (gdb) p mutex.lock.__data.__owner
  $1 = 15604

Upgrading to 2.21 seems to make these go away.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>