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 935e1d90 ("rust: pci:
require Send for Driver trait implementers") from the driver-core tree,
which fixes a missing concurrency requirement, and commit 9901adda
("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 <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/linux-next/20250324215702.1515ba92@canb.auug.org.au/

 [1]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20250324174048.1075597-1-ojeda@kernel.org


[ Added number to Closes. Fix typo spotted by Boqun. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>