mds: fix issues with use-after-free in C_Flush_Journal

Moved dout() statement out of trim_expired_segments() to avoid
dereferencing 'this' object after object deletion via context completion
handler.

Fixes: https://tracker.ceph.com/issues/69953
Signed-off-by: Milind Changire <[email protected]>

Author: mchangir

src/mds/MDSRank.cc

@@ -180,10 +180,6 @@ class C_Flush_Journal : public MDSInternalContext {
       handle_write_head(r);
     }));
     mdlog->trim_expired_segments(ctx);
-
-    dout(5) << __func__ << ": trimming is complete; wait for journal head write. Journal expire_pos/trim_pos is now "
-            << std::hex << mdlog->get_journaler()->get_expire_pos() << "/"
-            << mdlog->get_journaler()->get_trimmed_pos() << dendl;
   }
 
   void handle_write_head(int r) {
@@ -202,6 +198,10 @@ class C_Flush_Journal : public MDSInternalContext {
      */
     ceph_assert(ceph_mutex_is_locked_by_me(mds->mds_lock));
     dout(20) << __func__ << ": r=" << r << dendl;
+
+    dout(5) << __func__ << ": trimming is complete; wait for journal head write. Journal expire_pos/trim_pos is now "
+            << std::hex << mdlog->get_journaler()->get_expire_pos() << "/"
+            << mdlog->get_journaler()->get_trimmed_pos() << dendl;
     on_finish->complete(r);
   }