MDEV-24101 innodb_random_read_ahead=ON causes hang on DDL or shutdown

buf_read_ahead_random(): Do not leak a tablespace reference.
The reference was already acquired in fil_space_t::get(),
and we must only check that operations were not stopped.

This error was introduced when
commit 118e258a
merged n_pending_ios, n_pending_ops into a single n_pending.

This was not noticed earlier, because innodb_random_read_ahead
is OFF by default and our regression tests did not vary that
parameter at all.

mysql-test/suite/innodb/t/foreign_key.combinations

+[read-ahead]
+--innodb-random-read-ahead=ON
+
+[normal]
+--innodb-random-read-ahead=OFF

storage/innobase/buf/buf0rea.cc

@@ -431,7 +431,7 @@ buf_read_ahead_random(const page_id_t page_id, ulint zip_size, bool ibuf)
   return 0;
 
 read_ahead:
-  if (!space->acquire_if_not_stopped())
+  if (space->is_stopping())
     goto no_read_ahead;
 
   /* Read all the suitable blocks within the area */