MDEV-35409 InnoDB can still hang while running out of buffer pool

buf_pool_t::LRU_warn(): Also clear the try_LRU_scan flag, to ensure
that need_LRU_eviction() will hold. This should ensure progress when
buf_LRU_get_free_block() is expecting buf_flush_page_cleaner() to
make some room, even when buf_pool.LRU.count is small.

This hang was observed in trx_lists_init_at_db_start() while the last
batch of crash recovery was in progress, but it could theoretically
be possible also when a large part of the buffer pool is occupied by
record locks or the adaptive hash index.

Reviewed by: Debarun Banerjee

storage/innobase/buf/buf0flu.cc

@@ -2564,6 +2564,7 @@ static void buf_flush_page_cleaner()
 ATTRIBUTE_COLD void buf_pool_t::LRU_warn()
 {
   mysql_mutex_assert_owner(&mutex);
+  try_LRU_scan= false;
   if (!LRU_warned.test_and_set(std::memory_order_acquire))
     sql_print_warning("InnoDB: Could not free any blocks in the buffer pool!"
                       " %zu blocks are in use and %zu free."