rust: list: fix path of `assert_pinned!`

Commit dbd5058b ("rust: make pin-init its own crate") moved all
items from pin-init into the pin-init crate, including the
`assert_pinned!` macro.

Thus fix the path of the sole user of the `assert_pinned!` macro.

This occurrence was missed in the commit above, since it is in a macro
rule that has no current users (although binder is a future user).

Cc: stable@kernel.org
Fixes: dbd5058b ("rust: make pin-init its own crate")
Signed-off-by: Benno Lossin <lossin@kernel.org>
Link: https://lore.kernel.org/r/20250525173450.853413-1-lossin@kernel.org


[ Reworded slightly as discussed in the list. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>