fixup 40ab9c2c5e: deadlock

Reproduced by

test_runner/regress/test_branching.py::test_branching_with_pgbench[debug-pg16-flat-1-10]'

It kinda makes sense that this deadlocks in `sequential` mode.

However, it also deadlocks in `sidecar-task` mode.
I don't understand why.

pageserver/src/tenant/storage_layer/inmemory_layer.rs

@@ -470,7 +470,7 @@ impl InMemoryLayer {
                 }
             }
         }
-
+        drop(inner); // release the lock before we spawn the IO; if it's serial-mode IO we will deadlock on the read().await below
         let read_from = Arc::clone(self);
         let read_ctx = ctx.attached_child();
         reconstruct_state