net: dsa: tag_8021q: Future-proof the reserved fields in the custom VID

After witnessing the discussion in https://lkml.org/lkml/2019/8/14/151


w.r.t. ioctl extensibility, it became clear that such an issue might
prevent that the 3 RSV bits inside the DSA 802.1Q tag might also suffer
the same fate and be useless for further extension.

So clearly specify that the reserved bits should currently be
transmitted as zero and ignored on receive. The DSA tagger already does
this (and has always did), and is the only known user so far (no
Wireshark dissection plugin, etc). So there should be no incompatibility
to speak of.

Fixes: 0471dd42 ("net: dsa: tag_8021q: Create a stable binary format")
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>