rust: dma: add Send implementation for CoherentAllocation

Stephen found a future build failure in linux-next [1]:

    error[E0277]: `*mut MyStruct` cannot be sent between threads safely
      --> samples/rust/rust_dma.rs:47:22
       |
    47 | impl pci::Driver for DmaSampleDriver {
       |                      ^^^^^^^^^^^^^^^ `*mut MyStruct` cannot be sent between threads safely

It is caused by the interaction between commit 935e1d9 ("rust: pci:
require Send for Driver trait implementers") from the driver-core tree,
which fixes a missing concurrency requirement, and commit 9901add
("samples: rust: add Rust dma test sample driver") which adds a sample
that does not satisfy that requirement.

Add a `Send` implementation to `CoherentAllocation`, which allows the
sample (and other future users) to satisfy it.

Reported-by: Stephen Rothwell <[email protected]>
Closes: https://lore.kernel.org/linux-next/[email protected]/ [1]
Signed-off-by: Danilo Krummrich <[email protected]>
Reviewed-by: Boqun Feng <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Added number to Closes. Fix typo spotted by Boqun. - Miguel ]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: Danilo Krummrich

rust/kernel/dma.rs

@@ -301,6 +301,10 @@ impl<T: AsBytes + FromBytes> Drop for CoherentAllocation<T> {
     }
 }
 
+// SAFETY: It is safe to send a `CoherentAllocation` to another thread if `T`
+// can be sent to another thread.
+unsafe impl<T: AsBytes + FromBytes + Send> Send for CoherentAllocation<T> {}
+
 /// Reads a field of an item from an allocated region of structs.
 ///
 /// # Examples