dm: Add scaffolding to change completion function signature

This patch adds temporary scaffolding so that the Crypto API
completion function can take a void * instead of crypto_async_request.
Once affected users have been converted this can be removed.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>