rust: enable clippy::unnecessary_safety_doc lint

In Rust 1.67.0, Clippy added the `unnecessary_safety_doc` lint [1],
which is similar to `unnecessary_safety_comment`, but for `# Safety`
sections, i.e. safety preconditions in the documentation.

This is something that should not happen with our coding guidelines in
mind. Thus enable the lint to have it machine-checked.

Link: https://rust-lang.github.io/rust-clippy/master/index.html#/unnecessary_safety_doc [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

Makefile

@@ -458,6 +458,7 @@ export rust_common_flags := --edition=2021 \
 			    -Wclippy::no_mangle_with_rust_abi \
 			    -Wclippy::undocumented_unsafe_blocks \
 			    -Wclippy::unnecessary_safety_comment \
+			    -Wclippy::unnecessary_safety_doc \
 			    -Wrustdoc::missing_crate_level_docs
 
 KBUILD_HOSTCFLAGS   := $(KBUILD_USERHOSTCFLAGS) $(HOST_LFS_CFLAGS) \