MDEV-25404 fixup: Fix ssux_lock_low::u_wr_upgrade()

The U-to-X upgrade turned out to be incorrect. A debug assertion
failed in wr_wait(), called from mtr_defer_drop_ahi() in a stress
test with innodb_adaptive_hash_index=ON.

A correct upgrade procedure ought to be readers.fetch_add(WRITER-1)
to register ourselves as a WRITER (or waiting writer) and to release
the reference that was being held for the U lock.

Thanks to Matthias Leich for catching the problem.

storage/innobase/include/srw_lock.h

@@ -217,11 +217,9 @@ class ssux_lock_low final
   void u_wr_upgrade()
   {
     DBUG_ASSERT(writer.is_locked());
-    uint32_t lk= 1;
-    if (!readers.compare_exchange_strong(lk, WRITER,
-                                         std::memory_order_acquire,
-                                         std::memory_order_relaxed))
-      wr_wait(lk);
+    uint32_t lk= readers.fetch_add(WRITER - 1, std::memory_order_acquire);
+    if (lk != 1)
+      wr_wait(lk - 1);
   }
   void wr_u_downgrade()
   {