Both cxgbe(4) and mlx5(4) wrapped the existing send tag header with
their own identical headers that stored the type that the
type-specific tag structures inherited from, so in practice it seems
drivers need this in the tag anyway. This permits removing these
extra header indirections (struct cxgbe_snd_tag and struct
mlx5e_snd_tag).
In addition, this permits driver-independent code to query the type of
a tag, e.g. to know what type of tag is being queried via
if_snd_query.