seg6: add PSP flavor support for SRv6 End behavior

The "flavors" framework defined in RFC8986 [1] represents additional
operations that can modify or extend a subset of existing behaviors such as
SRv6 End, End.X and End.T. We report these flavors hereafter:
 - Penultimate Segment Pop (PSP);
 - Ultimate Segment Pop (USP);
 - Ultimate Segment Decapsulation (USD).

Depending on how the Segment Routing Header (SRH) has to be handled, an
SRv6 End* behavior can support these flavors either individually or in
combinations.
In this patch, we only consider the PSP flavor for the SRv6 End behavior.

A PSP enabled SRv6 End behavior is used by the Source/Ingress SR node
(i.e., the one applying the SRv6 Policy) when it needs to instruct the
penultimate SR Endpoint node listed in the SID List (carried by the SRH) to
remove the SRH from the IPv6 header.

Specifically, a PSP enabled SRv6 End behavior processes the SRH by:
   i) decreasing the Segment Left (SL) from 1 to 0;
  ii) copying the Last Segment IDentifier (SID) into the IPv6 Destination
      Address (DA);
 iii) removing (i.e., popping) the outer SRH from the extension headers
      following the IPv6 header.

It is important to note that PSP operation (steps i, ii, iii) takes place
only at a penultimate SR Segment Endpoint node (i.e., when the SL=1) and
does not happen at non-penultimate Endpoint nodes. Indeed, when a SID of
PSP flavor is processed at a non-penultimate SR Segment Endpoint node, the
PSP operation is not performed because it would not be possible to decrease
the SL from 1 to 0.

                                                 SL=2 SL=1 SL=0
                                                   |    |    |
For example, given the SRv6 policy (SID List := <  X,   Y,   Z  >):
 - a PSP enabled SRv6 End behavior bound to SID "Y" will apply the PSP
   operation as Segment Left (SL) is 1, corresponding to the Penultimate
   Segment of the SID List;
 - a PSP enabled SRv6 End behavior bound to SID "X" will *NOT* apply the
   PSP operation as the Segment Left is 2. This behavior instance will
   apply the "standard" End packet processing, ignoring the configured PSP
   flavor at all.

[1] - RFC8986: https://datatracker.ietf.org/doc/html/rfc8986Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>