wifi: mac80211: sta_info: fix link_sta insertion

When inserting a link STA, make sure it doesn't exist first
and add lockdep assertions that we cannot modify the hash
table without holding the sta_mtx, so this check is really
correct.

Also return without hashing if the driver failed, and warn
if the hashing fails, which shouldn't happen due to the
check described above.

Fixes: cb71f1d1 ("wifi: mac80211: add sta link addition/removal")
Fixes: ba6ddab9 ("wifi: mac80211: maintain link-sta hash table")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>