realm-io: fix bidi_zero_copy

[liusen373]

修复linux下失效的零拷贝函数bidi_zero_copy

系统调用总是失败，错误信息为：

splice(7, NULL, 10, NULL, 18446744073709551615, SPLICE_F_MOVE|SPLICE_F_NONBLOCK) = -1 EINVAL (Invalid argument)

原因是调用splice函数时，长度参数超过了内核最大限制，usize_t在强制转换成ssize_t时溢出
https://github.com/torvalds/linux/blob/c17b750b3ad9f45f2b6f7e6f7f4679844244f0b9/fs/splice.c#L1340
https://github.com/torvalds/linux/blob/c17b750b3ad9f45f2b6f7e6f7f4679844244f0b9/fs/read_write.c#L451-L457

[liusen373]

下面是一份简单的性能测试

在本机搭建 speedtest-rust 服务器，使用 librespeed-cli 作为客户端，realm 在回环接口上转发。

原版realm的性能（单线程）

下载

> ./librespeed-cli --local-json ./servers.json --server 1 --no-icmp --no-upload
Using local JSON server list: ./servers.json
Selected server: Rust Local [127.0.0.1]
You're testing from: 127.0.0.1 - localhost IPv4 access
Ping: 0.00 ms   Jitter: 0.00 ms
Download rate:  7388.81 Mbps
Upload test is disabled

上传

> ./librespeed-cli --local-json ./servers.json --server 1 --no-icmp --no-download
Using local JSON server list: ./servers.json
Selected server: Rust Local [127.0.0.1]
You're testing from: 127.0.0.1 - localhost IPv4 access
Ping: 0.00 ms   Jitter: 0.00 ms
Download test is disabled
Upload rate:    7391.85 Mbps

zero copy版realm的性能（单线程）

下载

> ./librespeed-cli --local-json ./servers.json --server 1 --no-icmp --no-upload
Using local JSON server list: ./servers.json
Selected server: Rust Local [127.0.0.1]
You're testing from: 127.0.0.1 - localhost IPv4 access
Ping: 0.00 ms   Jitter: 0.00 ms
Download rate:  38859.67 Mbps
Upload test is disabled

上传

> ./librespeed-cli --local-json ./servers.json --server 1 --no-icmp --no-download
Using local JSON server list: ./servers.json
Selected server: Rust Local [127.0.0.1]
You're testing from: 127.0.0.1 - localhost IPv4 access
Ping: 0.00 ms   Jitter: 0.00 ms
Download test is disabled
Upload rate:    12786.49 Mbps

注意：39Gbps/13Gbps 不是 realm 的性能极限，是speedtest-rust和librespeed-cli的性能极限

[zephyrchien]

LGTM. Thank you for the fix and detailed reasoning!