MDEV-23410 buf_LRU_scan_and_free_block() fails to stop at first freed block

An unfortunate performance regression was introduced in
commit b1ab211dee (MDEV-15053)
that affects workloads where the working set is larger than
the buffer pool.

We used to terminate the loop in buf_LRU_scan_and_free_block()
when we succeeded to free a replaceable buffer pool block.
Due to the regression, we would keep freeing all replaceable
blocks until we reach the end of the list or encounter a block
that had never been accessed because it had been brought in
by read-ahead.

storage/innobase/buf/buf0lru.cc

@@ -570,8 +570,9 @@ static bool buf_LRU_free_from_common_LRU_list(bool scan_all)
 				ever being accessed. This gives us a measure of
 				the effectiveness of readahead */
 				++buf_pool.stat.n_ra_pages_evicted;
-				break;
 			}
+
+			break;
 		}
 	}