rust: rbtree: fix SAFETY comments that should be # Safety sections

The tag `SAFETY` is used for safety comments, i.e. `// SAFETY`, while a
`Safety` section is used for safety preconditions in code documentation,
i.e. `/// # Safety`.

Fix the three instances recently added in `rbtree` that Clippy would
have normally caught in a public item, so that we can enable checking
of private items in one of the following commits.

Fixes: 98c14e4 ("rust: rbtree: add cursor")
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20240904204347.168520-14-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/kernel/rbtree.rs

@@ -884,7 +884,8 @@ impl<'a, K, V> Cursor<'a, K, V> {
         NonNull::new(neighbor)
     }
 
-    /// SAFETY:
+    /// # Safety
+    ///
     /// - `node` must be a valid pointer to a node in an [`RBTree`].
     /// - The caller has immutable access to `node` for the duration of 'b.
     unsafe fn to_key_value<'b>(node: NonNull<bindings::rb_node>) -> (&'b K, &'b V) {
@@ -894,7 +895,8 @@ impl<'a, K, V> Cursor<'a, K, V> {
         (k, unsafe { &*v })
     }
 
-    /// SAFETY:
+    /// # Safety
+    ///
     /// - `node` must be a valid pointer to a node in an [`RBTree`].
     /// - The caller has mutable access to `node` for the duration of 'b.
     unsafe fn to_key_value_mut<'b>(node: NonNull<bindings::rb_node>) -> (&'b K, &'b mut V) {
@@ -904,7 +906,8 @@ impl<'a, K, V> Cursor<'a, K, V> {
         (k, unsafe { &mut *v })
     }
 
-    /// SAFETY:
+    /// # Safety
+    ///
     /// - `node` must be a valid pointer to a node in an [`RBTree`].
     /// - The caller has immutable access to the key for the duration of 'b.
     unsafe fn to_key_value_raw<'b>(node: NonNull<bindings::rb_node>) -> (&'b K, *mut V) {