Address Copilot's review comments

Fix lambda reference capture bug where all threads processed the last disk's
stripes instead of their own; fix shared 0xdeadbeef EThread* sentinel by using
a thread_local variable to give each OS thread a unique identity for
MUTEX_TAKE_LOCK.

Author: zwoop

src/iocore/cache/CacheDir.cc

@@ -981,8 +981,11 @@ sync_cache_dir_on_shutdown()
   threads.reserve(drive_stripe_map.size());
   for (auto &[disk, indices] : drive_stripe_map) {
     Dbg(dbg_ctl_cache_dir_sync, "Disk %s: syncing %zu stripe(s)", disk->path, indices.size());
-    threads.emplace_back([&indices]() {
-      EThread *t = reinterpret_cast<EThread *>(0xdeadbeef);
+    threads.emplace_back([indices]() {
+      // Use a thread_local variable to give each OS thread a unique EThread* sentinel,
+      // satisfying MUTEX_TAKE_LOCK's thread_holding check without a real EThread.
+      thread_local char thread_sentinel;
+      EThread          *t = reinterpret_cast<EThread *>(&thread_sentinel);
 
       for (int idx : indices) {
         gstripes[idx]->shutdown(t);