According to the BER encoding rules, integer value should be encoded
as two's complement, and if the highest bit of a positive integer
is 1, should add a leading zero-octet.

The kernel's built-in RSA algorithm cannot recognize negative numbers
when parsing keys, so it can pass this test case.

Export the key to file and run the following command to verify the
fix result:

  openssl asn1parse -inform DER -in /path/to/key/file
Signed-off-by: Lei He <helei.sig11@bytedance.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>