crypto: lrw - Free rctx->ext with kzfree

The buffer rctx->ext contains potentially sensitive data and should
be freed with kzfree.

Cc: <stable@vger.kernel.org>
Fixes: 700cb3f5 ("crypto: lrw - Convert to skcipher")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>