Introduce NetlinkHeader::LEN

Signed-off-by: Gris Ge <fge@redhat.com>

Author: cathay4t

src/header.rs

@@ -39,6 +39,10 @@ pub struct NetlinkHeader {
     pub port_number: u32,
 }
 
+impl NetlinkHeader {
+    pub const LEN: &usize = NETLINK_HEADER_LEN;
+}
+
 impl Emitable for NetlinkHeader {
     fn buffer_len(&self) -> usize {
         NETLINK_HEADER_LEN