crypto: algif_skcipher - Cap recv SG list at ctx->used

Somewhere along the line the cap on the SG list length for receive
was lost.  This patch restores it and removes the subsequent test
which is now redundant.

Fixes: 2d97591e ("crypto: af_alg - consolidation of...")
Cc: <stable@vger.kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>