rust: debugfs: Implement Reader for Mutex<T> only when T is Unpin

rust: debugfs: Implement Reader for Mutex<T> only when T is Unpin

Since we are going to make `Mutex<T>` structurally pin the data (i.e.
`T`), therefore `.lock()` function only returns a `Guard` that can
dereference a mutable reference to `T` if only `T` is `Unpin`, therefore
restrict the impl `Reader` block of `Mutex<T>` to that.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20251022034237.70431-1-boqun.feng@gmail.com