mac80211: always allocate struct ieee802_11_elems

As the 802.11 spec evolves, we need to parse more and more
elements. This is causing the struct to grow, and we can no
longer get away with putting it on the stack.

Change the API to always dynamically allocate and return an
allocated pointer that must be kfree()d later.

As an alternative, I contemplated a scheme whereby we'd say
in the code which elements we needed, e.g.

    DECLARE_ELEMENT_PARSER(elems,
                           SUPPORTED_CHANNELS,
                           CHANNEL_SWITCH,
                           EXT(KEY_DELIVERY));

    ieee802_11_parse_elems(..., &elems, ...);

and while I think this is possible and will save us a lot
since most individual places only care about a small subset
of the elements, it ended up being a bit more work since a
lot of places do the parsing and then pass the struct to
other functions, sometimes with multiple levels.

Link: https://lore.kernel.org/r/20210920154009.26caff6b5998.I05ae58768e990e611aee8eca8abefd9d7bc15e05@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>