fix off-by-one error in range check of Backlog::new (#2500)

Author: fkm3

src/sys/socket/mod.rs

@@ -2190,7 +2190,7 @@ impl Backlog {
 
         let val = val.into();
 
-        if !(MIN..Self::MAXCONN.0).contains(&val) {
+        if !(MIN..=Self::MAXCONN.0).contains(&val) {
             return Err(Errno::EINVAL);
         }
 

test/sys/test_socket.rs

@@ -1695,6 +1695,13 @@ pub fn test_named_unixdomain() {
     assert_eq!(&buf[..], b"hello");
 }
 
+#[test]
+pub fn test_listen_maxbacklog() {
+    use nix::sys::socket::Backlog;
+
+    assert!(Backlog::new(libc::SOMAXCONN).is_ok());
+}
+
 #[test]
 pub fn test_listen_wrongbacklog() {
     use nix::sys::socket::Backlog;