seg6: improve management of behavior attributes

Depending on the attribute (i.e.: SEG6_LOCAL_SRH, SEG6_LOCAL_TABLE, etc),
the parse() callback performs some validity checks on the provided input
and updates the tunnel state (slwt) with the result of the parsing
operation. However, an attribute may also need to reserve some additional
resources (i.e.: memory or setting up an eBPF program) in the parse()
callback to complete the parsing operation.

The parse() callbacks are invoked by the parse_nla_action() for each
attribute belonging to a specific behavior. Given a behavior with N
attributes, if the parsing of the i-th attribute fails, the
parse_nla_action() returns immediately with an error. Nonetheless, the
resources acquired during the parsing of the i-1 attributes are not freed
by the parse_nla_action().

Attributes which acquire resources must release them *in an explicit way*
in both the seg6_local_{build/destroy}_state(). However, adding a new
attribute of this type requires changes to
seg6_local_{build/destroy}_state() to release the resources correctly.

The seg6local infrastructure still lacks a simple and structured way to
release the resources acquired in the parse() operations.

We introduced a new callback in the struct seg6_action_param named
destroy(). This callback releases any resource which may have been acquired
in the parse() counterpart. Each attribute may or may not implement the
destroy() callback depending on whether it needs to free some acquired
resources.

The destroy() callback comes with several of advantages:

 1) we can have many attributes as we want for a given behavior with no
    need to explicitly free the taken resources;

 2) As in case of the seg6_local_build_state(), the
    seg6_local_destroy_state() does not need to handle the release of
    resources directly. Indeed, it calls the destroy_attrs() function which
    is in charge of calling the destroy() callback for every set attribute.
    We do not need to patch seg6_local_{build/destroy}_state() anymore as
    we add new attributes;

 3) the code is more readable and better structured. Indeed, all the
    information needed to handle a given attribute are contained in only
    one place;

 4) it facilitates the integration with new features introduced in further
    patches.

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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