net_util: add safety comments for impl ByteValued

Signed-off-by: Wei Liu <liuwe@microsoft.com>

net_util/src/ctrl_queue.rs

@@ -42,6 +42,7 @@ pub struct ControlHeader {
     pub cmd: u8,
 }
 
+// SAFETY: ControlHeader only contains a series of integers
 unsafe impl ByteValued for ControlHeader {}
 
 pub struct CtrlQueue {

net_util/src/lib.rs

@@ -59,7 +59,7 @@ pub struct VirtioNetConfig {
     pub duplex: u8,
 }
 
-// Safe because it only has data and has no implicit padding.
+// SAFETY: it only has data and has no implicit padding.
 unsafe impl ByteValued for VirtioNetConfig {}
 
 /// Create a sockaddr_in from an IPv4 address, and expose it as