net_util: replace as <pointer> casts with safer alternatives

`as` casts can change mutability, which quickly leads to undefined
behavior.

Signed-off-by: Julian Schindel <mail@arctic-alpaca.de>

net_util/src/tap.rs

@@ -169,7 +169,7 @@ impl Tap {
             // Open calls are safe because we give a constant null-terminated
             // string and verify the result.
             libc::open(
-                c"/dev/net/tun".as_ptr() as *const c_char,
+                c"/dev/net/tun".as_ptr().cast(),
                 flags.unwrap_or(libc::O_RDWR | libc::O_NONBLOCK | libc::O_CLOEXEC),
             )
         };