nl80211: MBSSID and EMA support in AP mode

Add new attributes to configure support for multiple BSSID
and advanced multi-BSSID advertisements (EMA) in AP mode.

- NL80211_ATTR_MBSSID_CONFIG used for per interface configuration.
- NL80211_ATTR_MBSSID_ELEMS used to MBSSID elements for beacons.

Memory for the elements is allocated dynamically. This change frees
the memory in existing functions which call nl80211_parse_beacon(),
a comment is added to indicate the new references to do the same.

Signed-off-by: John Crispin <john@phrozen.org>
Co-developed-by: Aloka Dixit <alokad@codeaurora.org>
Signed-off-by: Aloka Dixit <alokad@codeaurora.org>
Link: https://lore.kernel.org/r/20210916025437.29138-2-alokad@codeaurora.org


[don't leave ERR_PTR hanging around]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>