X.509: unpack RSA signatureValue field from BIT STRING

The signatureValue field of a X.509 certificate is encoded as a BIT STRING.
For RSA signatures this BIT STRING is of so-called primitive subtype, which
contains a u8 prefix indicating a count of unused bits in the encoding.

We have to strip this prefix from signature data, just as we already do for
key data in x509_extract_key_data() function.

This wasn't noticed earlier because this prefix byte is zero for RSA key
sizes divisible by 8. Since BIT STRING is a big-endian encoding adding zero
prefixes has no bearing on its value.

The signature length, however was incorrect, which is a problem for RSA
implementations that need it to be exactly correct (like AMD CCP).

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Fixes: c26fd69f

 ("X.509: Add a crypto key parser for binary (DER) X.509 certificates")
Cc: stable@vger.kernel.org
Signed-off-by: James Morris <james.morris@microsoft.com>