MDEV-5830: Assertion failure mutex_get_waiters(mutex) == 0 at shutdown.

Analysis: XtraDB merge regression, at the end of mutex_spin_wait before goto mutex_loop
there is missing  

if (prio_mutex) { 
	os_atomic_decrement_ulint(&prio_mutex->high_priority_waiters, 1); 
}

Hence we get unbalanced waiter count.

Thanks to Laurynas Biveinis for finding this.

storage/xtradb/buf/buf0lru.cc

@@ -2290,14 +2290,11 @@ buf_LRU_block_free_non_file_page(
 		page_zip_set_size(&block->page.zip, 0);
 	}
 
-	//ut_ad(buf_pool->free_list_mutex.high_priority_waiters == 0);
 	mutex_enter_first(&buf_pool->free_list_mutex);
-	//ut_ad(buf_pool->free_list_mutex.high_priority_waiters == 0);
 	buf_block_set_state(block, BUF_BLOCK_NOT_USED);
 	UT_LIST_ADD_FIRST(list, buf_pool->free, (&block->page));
 	ut_d(block->page.in_free_list = TRUE);
 	mutex_exit(&buf_pool->free_list_mutex);
-	//ut_ad(buf_pool->free_list_mutex.high_priority_waiters == 0);
 
 	UNIV_MEM_ASSERT_AND_FREE(block->frame, UNIV_PAGE_SIZE);
 }

storage/xtradb/sync/sync0sync.cc

@@ -670,6 +670,12 @@ spin_loop:
 
 	sync_array_wait_event(sync_arr, index);
 
+	if (prio_mutex) {
+
+		os_atomic_decrement_ulint(&prio_mutex->high_priority_waiters,
+					  1);
+	}
+
 	goto mutex_loop;
 }