wifi: cfg80211: rewrite merging of inherited elements

The cfg80211_gen_new_ie function merges the IEs using inheritance rules.
Rewrite this function to fix issues around inheritance rules. In
particular, vendor elements do not require any special handling, as they
are either all inherited or overridden by the subprofile.
Also, add fragmentation handling as this may be needed in some cases.

This also changes the function to not require making a copy. The new
version could be optimized a bit by explicitly tracking which IEs have
been handled already rather than looking that up again every time.

Note that a small behavioural change is the removal of the SSID special
handling. This should be fine for the MBSSID element, as the SSID must
be included in the subelement.

Fixes: 0b8fb823 ("cfg80211: Parsing of Multiple BSSID information in scanning")
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230616094949.bc6152e146db.I2b5f3bc45085e1901e5b5192a674436adaf94748@changeid


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