librbd: ensure that thread pool lock is held when processing throttled IOs

librbd: ensure that thread pool lock is held when processing throttled IOs

There previously was a potential race for throttled IOs to complete prior
to the main worker thread finishing the processing of the blocked IO.

Fixes: https://tracker.ceph.com/issues/47371
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 2d86e0935aa6f0c392df428676d9ab0a338fccae)

Conflicts:
    src/test/librbd/io/test_mock_ImageRequestWQ.cc
- in Octopus, commit 792d6c53fedc695199cc18916347c1b545fe42c2 did a global
  replace of Mutex to ceph::mutex, so to fix this for Nautilus, we just need to
  do that in test_mock_ImageRequestWQ.cc since the get_pool_lock() method is
  returning a Mutex instead of a ceph::mutex