dcb: Generalize dcb_set_attribute()

dcb: Generalize dcb_set_attribute()

The function dcb_set_attribute() takes a fully-formed payload as an
argument. For callers that need to build a nested attribute, such as is the
case for DCB APP table, this is not great, because with libmnl, they would
need to construct a separate netlink message just to pluck out the payload
and hand it over to this function.

Currently, dcb_set_attribute() also always wraps the payload in an
DCB_ATTR_IEEE container, because that is what all the dcb subtools so far
needed. But that is not appropriate for DCBX in particular, and in fact a
handful other attributes, as well as any CEE payloads.

Instead, generalize this code by adding parameters for constructing a
custom payload and for fetching the response from a custom response
attribute. Then add dcb_set_attribute_va(), which takes a callback to
invoke in the right place for the nest to be built, and
dcb_set_attribute_bare(), which is similar to dcb_set_attribute(), but does
not encapsulate the payload in an IEEE container. Rewrite
dcb_set_attribute() compatibly in terms of the new functions.

Signed-off-by: Petr Machata <me@pmachata.org>
Signed-off-by: David Ahern <dsahern@kernel.org>