librbd: fix a crash in get_rollback_snap_id

get_rollback_snap_id() did not check if the snapshot it was
accessing was a mirror snapshot, causing it to crash if it wasn't.

Fixes: https://tracker.ceph.com/issues/70075

Signed-off-by: N Balachandran <[email protected]>

Author: nbalacha

src/librbd/mirror/snapshot/Utils.cc

@@ -30,12 +30,15 @@ bool get_rollback_snap_id(
     uint64_t *rollback_snap_id) {
 
   for (; it != end; it++) {
-    auto mirror_ns = std::get<cls::rbd::MirrorSnapshotNamespace>(
-      it->second.snap_namespace);
-    if (mirror_ns.state != cls::rbd::MIRROR_SNAPSHOT_STATE_NON_PRIMARY) {
+    auto mirror_ns = std::get_if<cls::rbd::MirrorSnapshotNamespace>(
+      &it->second.snap_namespace);
+    if (mirror_ns == nullptr) {
+      continue;
+    }
+    if (mirror_ns->state != cls::rbd::MIRROR_SNAPSHOT_STATE_NON_PRIMARY) {
       break;
     }
-    if (mirror_ns.complete) {
+    if (mirror_ns->complete) {
       break;
     }
   }