wifi: mac80211: improve station iteration ergonomics

wifi: mac80211: improve station iteration ergonomics

Right now, the only way to iterate stations is to declare an
iterator function, possibly data structure to use, and pass all
that to the iteration helper function. This is annoying, and
there's really no inherent need for it.

Add a new for_each_station() macro that does the iteration in
a more ergonomic way. To avoid even more exported functions, do
the old ieee80211_iterate_stations_mtx() as an inline using the
new way, which may also let the compiler optimise it a bit more,
e.g. via inlining the iterator function.

Link: https://patch.msgid.link/20260108143431.d2b641f6f6af.I4470024f7404446052564b15bcf8b3f1ada33655@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>