crypto: rsa - only require output buffers as big as needed.

rhe RSA operations explicitly left-align the integers being written
skipping any leading zero bytes, but still require the output buffers to
include just enough space for the integer + the leading zero bytes.
Since the size of integer + the leading zero bytes (i.e. the key modulus
size) can now be obtained more easily through crypto_akcipher_maxsize
change the operations to only require as big a buffer as actually needed
if the caller has that information.  The semantics for request->dst_len
don't change.
Signed-off-by: Andrew Zaborowski <andrew.zaborowski@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/rsa.c

@@ -91,12 +91,6 @@ static int rsa_enc(struct akcipher_request *req)
 		goto err_free_c;
 	}
 
-	if (req->dst_len < mpi_get_size(pkey->n)) {
-		req->dst_len = mpi_get_size(pkey->n);
-		ret = -EOVERFLOW;
-		goto err_free_c;
-	}
-
 	ret = -ENOMEM;
 	m = mpi_read_raw_from_sgl(req->src, req->src_len);
 	if (!m)
@@ -136,12 +130,6 @@ static int rsa_dec(struct akcipher_request *req)
 		goto err_free_m;
 	}
 
-	if (req->dst_len < mpi_get_size(pkey->n)) {
-		req->dst_len = mpi_get_size(pkey->n);
-		ret = -EOVERFLOW;
-		goto err_free_m;
-	}
-
 	ret = -ENOMEM;
 	c = mpi_read_raw_from_sgl(req->src, req->src_len);
 	if (!c)
@@ -180,12 +168,6 @@ static int rsa_sign(struct akcipher_request *req)
 		goto err_free_s;
 	}
 
-	if (req->dst_len < mpi_get_size(pkey->n)) {
-		req->dst_len = mpi_get_size(pkey->n);
-		ret = -EOVERFLOW;
-		goto err_free_s;
-	}
-
 	ret = -ENOMEM;
 	m = mpi_read_raw_from_sgl(req->src, req->src_len);
 	if (!m)
@@ -225,12 +207,6 @@ static int rsa_verify(struct akcipher_request *req)
 		goto err_free_m;
 	}
 
-	if (req->dst_len < mpi_get_size(pkey->n)) {
-		req->dst_len = mpi_get_size(pkey->n);
-		ret = -EOVERFLOW;
-		goto err_free_m;
-	}
-
 	ret = -ENOMEM;
 	s = mpi_read_raw_from_sgl(req->src, req->src_len);
 	if (!s) {