wifi: mac80211: start building elements in SKBs

The building of elements is really mess, and really the only
reason we're not doing it in SKBs in the first place is that
the scan code in ieee80211_build_preq_ies() doesn't.

Convert ieee80211_build_preq_ies() to use an SKB internally
so that we can gradually convert other things to ..._put_*()
style interfaces.

Link: https://msgid.link/20240129202041.c3a8e3c2cc99.I9d9920858c30ae5154719783933de0d7bc2a2cb9@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>