rust: sync: atomic: Remove bound T: Sync for Atomic::from_ptr()

Originally, `Atomic::from_ptr()` requires `T` being a `Sync` because I
thought having the ability to do `from_ptr()` meant multiplle
`&Atomic<T>`s shared by different threads, which was identical (or
similar) to multiple `&T`s shared by different threads. Hence `T` was
required to be `Sync`. However this is not true, since `&Atomic<T>` is
not the same at `&T`. Moreover, having this bound makes `Atomic::<*mut
T>::from_ptr()` impossible, which is definitely not intended. Therefore
remove the `T: Sync` bound.

[boqun: Fix title typo spotted by Alice & Gary]

Fixes: 29c32c405e ("rust: sync: atomic: Add generic atomics")
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260120115207.55318-2-boqun.feng@gmail.com
Link: https://patch.msgid.link/20260303201701.12204-2-boqun@kernel.org

rust/kernel/sync/atomic.rs

@@ -204,10 +204,7 @@ pub const fn new(v: T) -> Self {
     /// // no data race.
     /// unsafe { Atomic::from_ptr(foo_a_ptr) }.store(2, Release);
     /// ```
-    pub unsafe fn from_ptr<'a>(ptr: *mut T) -> &'a Self
-    where
-        T: Sync,
-    {
+    pub unsafe fn from_ptr<'a>(ptr: *mut T) -> &'a Self {
         // CAST: `T` and `Atomic<T>` have the same size, alignment and bit validity.
         // SAFETY: Per function safety requirement, `ptr` is a valid pointer and the object will
         // live long enough. It's safe to return a `&Atomic<T>` because function safety requirement