crypto: asymmetric_keys - Fix unaligned access in x509_get_sig_params()

x509_get_sig_params() has the same code pattern as the one in
pkcs7_verify() that is fixed by commit 62f57d05 ("crypto: pkcs7 - Fix
unaligned access in pkcs7_verify()") so apply a similar fix here: make
sure that desc is pointing at an algined value past the digest_size,
and take alignment values into consideration when doing kzalloc()

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>