library: Normalize safety-for-unsafe-block comments

Almost all safety comments are of the form `// SAFETY:`,
so normalize the rest and fix a few of them that should
have been a `/// # Safety` section instead.

Furthermore, make `tidy` only allow the uppercase form. While
currently `tidy` only checks `core`, it is a good idea to prevent
`core` from drifting to non-uppercase comments, so that later
we can start checking `alloc` etc. too.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

src/tools/tidy/src/style.rs

@@ -289,7 +289,7 @@ pub fn check(path: &Path, bad: &mut bool) {
                     suppressible_tidy_err!(err, skip_undocumented_unsafe, "undocumented unsafe");
                 }
             }
-            if line.contains("// SAFETY:") || line.contains("// Safety:") {
+            if line.contains("// SAFETY:") {
                 last_safety_comment = true;
             } else if line.trim().starts_with("//") || line.trim().is_empty() {
                 // keep previous value