pageserver: don't try to flush if shutdown during attach (#8235)

## Problem

test_location_conf_churn fails on log errors when it tries to shutdown a
pageserver immediately after starting a tenant attach, like this:
https://neon-github-public-dev.s3.amazonaws.com/reports/pr-8224/9761000525/index.html#/testresult/15fb6beca5c7327c

```
shutdown:shutdown{tenant_id=35f5c55eb34e7e5e12288c5d8ab8b909 shard_id=0000}:timeline_shutdown{timeline_id=30936747043353a98661735ad09cbbfe shutdown_mode=FreezeAndFlush}: failed to freeze and flush: cannot flush frozen layers when flush_loop is not running, state is Exited\n')
```

This is happening because Tenant::shutdown fires its cancellation token
early if the tenant is not fully attached by the time shutdown is
called, so the flush loop is shutdown by the time we try and flush.

## Summary of changes

- In the early-cancellation case, also set the shutdown mode to Hard to
skip trying to do a flush that will fail.

pageserver/src/tenant.rs

@@ -1816,9 +1816,15 @@ impl Tenant {
         // If we're still attaching, fire the cancellation token early to drop out: this
         // will prevent us flushing, but ensures timely shutdown if some I/O during attach
         // is very slow.
-        if matches!(self.current_state(), TenantState::Attaching) {
+        let shutdown_mode = if matches!(self.current_state(), TenantState::Attaching) {
             self.cancel.cancel();
-        }
+
+            // Having fired our cancellation token, do not try and flush timelines: their cancellation tokens
+            // are children of ours, so their flush loops will have shut down already
+            timeline::ShutdownMode::Hard
+        } else {
+            shutdown_mode
+        };
 
         match self.set_stopping(shutdown_progress, false, false).await {
             Ok(()) => {}