Store the send tag type in the common send tag header.

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.

Reviewed by: gallatin, hselasky, np, kib
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D26689