crypto: virtio - Erase some sensitive memory when it is freed

virtcrypto_clear_request() does the same as the code here, but uses
kfree_sensitive() for one of the free operation.

So, better safe than sorry, use virtcrypto_clear_request() directly to
save a few lines of code and cleanly free the memory.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Tested-by: Lei Yang <leiyang@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>