rust: arc: add explicit `drop()` around `Box::from_raw()`

`Box::from_raw()` is `#[must_use]`, which means the result cannot
go unused.

In Rust 1.71.0, this was not detected because the block expression
swallows the diagnostic [1]:

    unsafe { Box::from_raw(self.ptr.as_ptr()) };

It would have been detected, however, if the line had been instead:

    unsafe { Box::from_raw(self.ptr.as_ptr()); }

i.e. the semicolon being inside the `unsafe` block, rather than
outside.

In Rust 1.72.0, the compiler started warning about this [2], so
without this patch we will get:

        error: unused return value of `alloc::boxed::Box::<T>::from_raw` that must be used
        --> rust/kernel/sync/arc.rs:302:22
        |
    302 |             unsafe { Box::from_raw(self.ptr.as_ptr()) };
        |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: call `drop(Box::from_raw(ptr))` if you intend to drop the `Box`
        = note: `-D unused-must-use` implied by `-D warnings`
    help: use `let _ = ...` to ignore the resulting value
        |
    302 |             unsafe { let _ = Box::from_raw(self.ptr.as_ptr()); };
        |                      +++++++                                 +

Thus add an add an explicit `drop()` as the `#[must_use]`'s
annotation suggests (instead of the more general help line).

Link: https://github.com/rust-lang/rust/issues/104253 [1]
Link: https://github.com/rust-lang/rust/pull/112529 [2]
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Reviewed-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
Link: https://lore.kernel.org/r/20230823160244.188033-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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