osd/PrimaryLogPG: Avoid accessing destroyed references in finish_degraded_object

osd/PrimaryLogPG: Avoid accessing destroyed references in finish_degraded_object

As follows:
for (auto i = callbacks_for_degraded_object.begin(); i != callbacks_for_degraded_object.end();) {
    finish_degraded_object((i++)->first);
}

void PrimaryLogPG::finish_degraded_object(const hobject_t oid)
{
  if (callbacks_for_degraded_object.count(oid)) {
    contexts.swap(callbacks_for_degraded_object[oid]);
    callbacks_for_degraded_object.erase(oid);   // Release
  }

  map<hobject_t, snapid_t>::iterator i = objects_blocked_on_degraded_snap.find(
    oid.get_head());  // Access
  ...
}

Fixes: https://tracker.ceph.com/issues/41250
Signed-off-by: Tao Ning <ningtao@sangfor.com.cn>
(cherry picked from commit 86d55c1a0ddb48efc0c1934728d27f22cf49dfa1)

Conflicts:
	src/osd/PrimaryLogPG.h
- retained "override" in nautilus
- PrimaryLogPG derives from PGBackend::Listener in nautilus, and its
  PGBackend::Listener's only derived class. So we need to update
  PGBackend::Listener accordingly. (quoted from Kefu's mimic backport)
  NOTE: This is needed because 8a8947d2a32d6390cb17099398e7f2212660c9a1 went in
  post-nautilus and is not being backported.