rust: list: Add unsafe blocks for container_of and safety comments

impl_list_item_mod.rs calls container_of! without unsafe blocks at a
couple of places. Since container_of! is unsafe, the blocks are strictly
necessary.

The problem was so far not visible because the "unsafe-op-in-unsafe-fn"
check is a lint rather than a hard compiler error, and Rust suppresses
lints triggered inside of a macro from another crate.

Thus, the error becomes only visible once someone from within the kernel
crate tries to use linked lists:

error[E0133]: call to unsafe function `core::ptr::mut_ptr::<impl *mut T>::byte_sub`
is unsafe and requires unsafe block
--> rust/kernel/lib.rs:252:29
|
252 | let container_ptr = field_ptr.byte_sub(offset).cast::<$Container>();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
|
::: rust/kernel/drm/jq.rs:98:1
|
98 | / impl_list_item! {
99 | | impl ListItem<0> for BasicItem { using ListLinks { self.links }; }
100 | | }
| |_- in this macro invocation
|
note: an unsafe function restricts its caller, but its body is safe by default
--> rust/kernel/list/impl_list_item_mod.rs:216:13
|
216 | unsafe fn view_value(me: *mut $crate::list::ListLinks<$num>) -> *const Self {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
::: rust/kernel/drm/jq.rs:98:1
|
98 | / impl_list_item! {
99 | | impl ListItem<0> for BasicItem { using ListLinks { self.links }; }
100 | | }
| |_- in this macro invocation
= note: requested on the command line with `-D unsafe-op-in-unsafe-fn`
= note: this error originates in the macro `$crate::container_of` which comes
from the expansion of the macro `impl_list_item`

Therefore, add unsafe blocks to container_of! calls to fix the issue.

[ As discussed, let's fix the build for those that want to use the
macro within the `kernel` crate now and we can discuss the proper
safety comments afterwards. Thus I removed the ones from the patch.

However, we cannot just avoid the comments with `CLIPPY=1`, so I
provided placeholders for now, like we did in the past. They were
also needed for an `unsafe impl`.

While I am not happy about it, it isn't worse than the current
status (the comments were meant to be there), and at least this
shows what is missing -- our pre-existing "good first issue" [1]
may motivate new contributors to complete them properly.

Finally, I moved one of the existing safety comments one line down
so that Clippy could locate it.

Link: https://github.com/Rust-for-Linux/linux/issues/351 [1]

- Miguel ]

Cc: stable@vger.kernel.org
Fixes: c77f85b347dd ("rust: list: remove OFFSET constants")
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260216131613.45344-3-phasta@kernel.org
[ Fixed formatting. Reworded to fix the lint suppression
explanation. Indent build error. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>