rust: sync: require `T: Sync` for `LockedBy::access`

The `LockedBy::access` method only requires a shared reference to the
owner, so if we have shared access to the `LockedBy` from several
threads at once, then two threads could call `access` in parallel and
both obtain a shared reference to the inner value. Thus, require that
`T: Sync` when calling the `access` method.

An alternative is to require `T: Sync` in the `impl Sync for LockedBy`.
This patch does not choose that approach as it gives up the ability to
use `LockedBy` with `!Sync` types, which is okay as long as you only use
`access_mut`.

Cc: stable@vger.kernel.org
Fixes: 7b1f55e3a984 ("rust: sync: introduce `LockedBy`")
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20240915-locked-by-sync-fix-v2-1-1a8d89710392@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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