seccomp: drop misleading argument name inherited from the OCI spec

seccomp: drop misleading argument name inherited from the OCI spec

The last (optional) argument was named "valueTwo", which seems to
originate from the OCI runtime spec:
https://github.com/opencontainers/runtime-spec/blob/master/config-linux.md#seccomp

In proper seccomp terminology, "value" is "datum_a" and "valueTwo" is "datum_b".

However, LXC's "valueTwo" was used as the mask for SCMP_CMP_MASKED_EQ,
while the mask is supposed to be "datum_a".

Signed-off-by: Felix Abecassis <fabecassis@nvidia.com>