Avoid using magic number in test

Author: Gleb Pomykalov

test/sys/test_socket.rs

@@ -249,7 +249,9 @@ mod recvfrom {
                                None
             ).unwrap();
 
-            setsockopt(rsock, UdpGsoSegment, &2).expect("setsockopt UDP_SEGMENT failed");
+            let segment_size = 2;
+
+            setsockopt(rsock, UdpGsoSegment, &segment_size).expect("setsockopt UDP_SEGMENT failed");
 
             bind(rsock, &sock_addr).unwrap();
             let ssock = socket(
@@ -259,7 +261,7 @@ mod recvfrom {
                 None,
             ).expect("send socket failed");
 
-            let from = sendrecv(rsock, ssock, Some(2), move |s, m, flags| {
+            let from = sendrecv(rsock, ssock, Some(segment_size), move |s, m, flags| {
                 let iov = [IoVec::from_slice(m)];
                 let cmsg = ControlMessage::UdpGsoSegments(&2);
                 sendmsg(s, &iov, &[cmsg], flags, Some(&sock_addr))