pkcs7: return correct error code if pkcs7_check_authattrs() fails

If pkcs7_check_authattrs() returns an error code, we should pass that
error code on, rather than using ENOMEM.

Fixes: 99db4435

 ("PKCS#7: Appropriately restrict authenticated attributes and content type")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: James Morris <james.l.morris@oracle.com>