wifi: cfg80211: say why the auth/assoc BSS lookup failed

The BSS lookup for an authentication or association request can fail
for three distinct reasons: cfg80211 has no scan entry at all for the
BSSID/channel, an entry exists but is older than
IEEE80211_SCAN_RESULT_EXPIRE (and not held), or a fresh entry exists
but its use_for flags do not allow this use. All three currently
surface as the same generic extack message "Error fetching BSS for
link" on the MLO association path, and as a bare -ENOENT with no
message at all on the authentication and non-MLO association paths.

Since wpa_supplicant logs the extack message verbatim ("nl80211:
kernel reports: ..."), that message is often the only diagnostic a
user sees when an MLO association degrades to fewer links, and it
does not say whether a fresh scan could have helped. In practice the
expired case is common for MLO partner links: 6 GHz is passive-scan
in many regulatory domains, so the partner-link entry is routinely
stale by the time userspace requests the association even though the
link is perfectly usable.

Let __cfg80211_get_bss() take an optional extack and record, during
the same bss_lock walk that fails the lookup, whether any matching
entry was rejected for being expired or for not being usable for the
requested use, and set a distinct message for each case (and a
combined one when different entries were rejected for different
reasons). Reorder the checks in the walk so that an entry's identity
(type, privacy, channel, BSSID/SSID) is established before the
usability checks; this doesn't change which entry is returned since
an entry is only used when all checks pass.

Also give the -EINVAL paths in nl80211_assoc_bss() proper messages
while at it, and keep pointing the bad_attr at the failing link on
the MLO path there; the message for that case is already set by the
lookup itself.

Signed-off-by: Louis Kotze <loukot@gmail.com>
Link: https://patch.msgid.link/20260722070734.3612581-2-loukot@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

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