[CRYPTO]: Fix stack overrun in crypt().

The stack allocation in crypt() is bogus as whether tmp_src/tmp_dst
is used is determined by factors unrelated to nbytes and
src->length/dst->length.

Since the condition for whether tmp_src/tmp_dst are used is very
complex, let's allocate them always instead of guessing.

This fixes a number of weird crashes including those AES crashes
that people have been seeing with the 2.4 backport + ipt_conntrack.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: James Morris <jmorris@redhat.com>
Signed-off-by: David S. Miller <davem@redhat.com>

crypto/cipher.c

@@ -52,8 +52,8 @@ static int crypt(struct crypto_tfm *tfm,
 {
 	struct scatter_walk walk_in, walk_out;
 	const unsigned int bsize = crypto_tfm_alg_blocksize(tfm);
-	u8 tmp_src[nbytes > src->length ? bsize : 0];
-	u8 tmp_dst[nbytes > dst->length ? bsize : 0];
+	u8 tmp_src[bsize];
+	u8 tmp_dst[bsize];
 
 	if (!nbytes)
 		return 0;