rust: dma: add dma_handle_with_offset method to CoherentAllocation

rust: dma: add dma_handle_with_offset method to CoherentAllocation

Sometimes one may want to obtain a DMA handle starting at a given
offset. This can be done by adding said offset to the result of
`dma_handle()`, but doing so on the client side carries the risk that
the operation will go outside the bounds of the allocation.

Thus, add a `dma_handle_with_offset` method that adds the desired offset
after checking that it is still valid.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250619-nova-frts-v6-3-ecf41ef99252@nvidia.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>