rust: macros: provide correct provenance when constructing THIS_MODULE

Currently while defining `THIS_MODULE` symbol in `module!()`, the
pointer used to construct `ThisModule` is derived from an immutable
reference of `__this_module`, which means the pointer doesn't have
the provenance for writing, and that means any write to that pointer
is UB regardless of data races or not. However, the usage of
`THIS_MODULE` includes passing this pointer to functions that may write
to it (probably in unsafe code), and this will create soundness issues.

One way to fix this is using `addr_of_mut!()` but that requires the
unstable feature "const_mut_refs". So instead of `addr_of_mut()!`,
an extern static `Opaque` is used here: since `Opaque<T>` is transparent
to `T`, an extern static `Opaque` will just wrap the C symbol (defined
in a C compile unit) in an `Opaque`, which provides a pointer with
writable provenance via `Opaque::get()`. This fix the potential UBs
because of pointer provenance unmatched.

Reported-by: Alice Ryhl <[email protected]>
Signed-off-by: Boqun Feng <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Reviewed-by: Trevor Gross <[email protected]>
Reviewed-by: Benno Lossin <[email protected]>
Reviewed-by: Gary Guo <[email protected]>
Closes: https://rust-for-linux.zulipchat.com/#narrow/stream/x/topic/x/near/465412664
Fixes: 1fbde52bde73 ("rust: add `macros` crate")
Cc: [email protected] # 6.6.x: be2ca1e03965: ("rust: types: Make Opaque::get const")
Link: https://lore.kernel.org/r/[email protected]
[ Fixed two typos, reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <[email protected]>

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