数据结构WriteRequest的size为64字节时assert才会成功

不同的处理器架构下，cacheline的size不一定都是64字节。
src/brpc/socket.cpp文件：
将代码
BAIDU_CASSERT(sizeof(WriteRequest) == 64, sizeof_write_request_is_64);
改成
BAIDU_CASSERT(sizeof(WriteRequest) == BAIDU_CACHELINE_SIZE, sizeof_write_request_is_cacheline_size);
是否更合适。

Open src/brpc/socket.cpp and inspect the WriteRequest size assertion; then check how BAIDU_CACHELINE_SIZE is defined for supported architectures. Done means the assertion uses the cache-line-size constant and the project builds successfully with the change.

Written by the indexing model from the issue text.