test: Create ParallelPGMapper object before start threadpool

In the case of creating a ParallelPGMapper object after
starting the threadpool the test case might crash.
The test case is crash while worker thread is trying to
delete a job but the object is still in the construction phase
not fully constructed.

Solution: To avoid a crash create the object before start
          a threadpool.

Fixes: https://tracker.ceph.com/issues/42884
Signed-off-by: Mohit Agrawal <[email protected]>

Author: mohit84

src/test/osd/TestOSDMap.cc

@@ -308,8 +308,8 @@ class OSDMapTest : public testing::Test,
     int cpu_num = 8;
     int pgs_per_chunk = 256;
     ThreadPool tp(cct, "BUG_40104::clean_upmap_tp", "clean_upmap_tp", cpu_num);
-    tp.start();
     ParallelPGMapper mapper(cct, &tp);
+    tp.start();
     vector<pg_t> pgs_to_check;
     om.get_upmap_pgs(&pgs_to_check);
     OSDMonitor::CleanUpmapJob job(cct, om, pending_inc);