Unique<T>: Data race allowed on T

[kuzeyardabulut]

Hi,
I found a memory-safety/soundness issue in this crate while scanning Rust code for potential vulnerabilities. This PR contains a fix for the issue.

Issue Description

Unique<T> unconditionally implements Sync. This allows users to create data races on T: !Sync. Such data races can lead to undefined behavior.

kernel/src/common/unique.rs

Lines 15 to 16 in 7269f9c

	unsafe impl<T> Send for Unique<T> {}
	unsafe impl<T> Sync for Unique<T> {}