rust: rbtree: fix comments referring to Box instead of KBox

Several safety comments in the RBTree implementation still refer to
"Box::from_raw" and "Box::into_raw", but the code actually uses KBox.
These comments were not updated when the implementation transitioned
from using Box to KBox.

Fixes: 8373147c ("rust: treewide: switch to our kernel `Box` type")
Signed-off-by: Charalampos Mitrodimas <charmitro@posteo.net>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250315-rbtree-comment-fixes-v1-1-51f72c420ff0@posteo.net


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