ossl_bn_mask_bits_fixed_top
ret = ossl_bn_mask_bits_fixed_top(a, n);
ossl_bn_mask_bits_fixed_top(r, n);
ossl_bn_mask_bits_fixed_top(out, BN_num_bits(range));
ossl_bn_mask_bits_fixed_top(out, blen_bits);
int ossl_bn_mask_bits_fixed_top(BIGNUM *a, int n);