In the latest version of the IPv6 Segment Routing IETF draft [1] the
cleanup flag is removed and the flags field length is shrunk from 16 bits
to 8 bits. As a consequence, the input of the HMAC computation is modified
in a non-backward compatible way by covering the whole octet of flags
instead of only the cleanup bit. As such, if an implementation compatible
with the latest draft computes the HMAC of an SRH who has other flags set
to 1, then the HMAC result would differ from the current implementation.

This patch carries those modifications to prevent conflict with other
implementations of IPv6 SR.

[1] https://tools.ietf.org/html/draft-ietf-6man-segment-routing-header-05

Signed-off-by: David Lebrun <david.lebrun@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>

Implement minimal support for processing of SR-enabled packets
as described in
https://tools.ietf.org/html/draft-ietf-6man-segment-routing-header-02.

This patch implements the following operations:
- Intermediate segment endpoint: incrementation of active segment and rerouting.
- Egress for SR-encapsulated packets: decapsulation of outer IPv6 header + SRH
  and routing of inner packet.
- Cleanup flag support for SR-inlined packets: removal of SRH if we are the
  penultimate segment endpoint.

A per-interface sysctl seg6_enabled is provided, to accept/deny SR-enabled
packets. Default is deny.

This patch does not provide support for HMAC-signed packets.

Signed-off-by: David Lebrun <david.lebrun@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>