crypto: api - Ensure cra_type->destroy is done in process context

Move the cra_type->destroy call out of crypto_alg_put and into
crypto_unregister_alg and crypto_free_instance.  This ensures
that it's always done in process context so calls such as flush_work
can be done.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>