mbuf: add a way to mark flowid as calculated from the internal headers

In some settings offload might calculate hash from decapsulated packet.
Reserve a bit in packet header rsstype to indicate that.

Add m_adj_decap() that acts similarly to m_adj, but also either clear
flowid if it is not marked as inner, or transfer it to the decapsulated
header, clearing inner indicator. It depends on the internals of m_adj()
that reuses the argument packet header for the result.

Use m_adj_decap() for decapsulating vxlan(4) and gif(4) input packets.

Reviewed by: ae, hselasky, np
Sponsored by: Nvidia Networking / Mellanox Technologies
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28773