... as noted by the compiler...

Fixes: 2cf492f ("examples/rust: Fix some new rustc warnings")
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

With at least

  rustc 1.79.0 (129f3b996 2024-06-10) (Fedora 1.79.0-3.fc40)

We were getting warnings when building the rust examples like

warning: creating a mutable reference to mutable static is discouraged
  --> src/lib.rs:75:40
   |
75 |     let ctx: *mut luw_ctx_t = unsafe { &mut CTX };
   |                                        ^^^^^^^^ mutable reference to mutable static
   |
   = note: for more information, see issue #114447 <https://github.com/rust-lang/rust/issues/114447>
   = note: this will be a hard error in the 2024 edition
   = note: this mutable reference has lifetime `'static`, but if the static gets accessed (read or written) by any other means, or any other reference is created, then any further use of this mutable reference is Undefined Behavior
   = note: `#[warn(static_mut_refs)]` on by default
help: use `addr_of_mut!` instead to create a raw pointer
   |
75 |     let ctx: *mut luw_ctx_t = unsafe { addr_of_mut!(CTX) };
   |                                        ~~~~~~~~~~~~~~~~~

So do like it says and use the addr_of_mut!() macro.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

We can put the unsafe keyword as part of the function definition,
getting rid of the unsafe {} blocks in the functions themselves.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

The programs demonstrate handling requests with payloads larger than
4GiB which means they need to be written out to disk and so also
demonstrates the use of the file-system access mechanism.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>