Do not inline transmit headers and use HW VLAN tagging if supported by mlx5en(4).

Query the minimal inline mode supported by the card.
When creating a send queue, cache the queried mode and optimize the transmit
if no inlining is required. In this case, we can avoid touching the headers
cache line and avoid dirtying several more lines by copying headers into
the send WQEs. Also, if no inline headers are used, hardware assists in
the VLAN tag framing.

Submitted by: kib@, slavash@
MFC after: 1 week
Sponsored by: Mellanox Technologies