cfg80211: comprehensively check station changes

The station change API isn't being checked properly before
drivers are called, and as a result it is difficult to see
what should be allowed and what not.

In order to comprehensively check the API parameters parse
everything first, and then have the driver call a function
(cfg80211_check_station_change()) with the additionally
information about the kind of station that is being changed;
this allows the function to make better decisions than the
old code could.

While at it, also add a few checks, particularly in mesh
and clarify the TDLS station lifetime in documentation.

To be able to reduce a few checks, ignore any flag set bits
when the mask isn't set, they shouldn't be applied then.

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

include/uapi/linux/nl80211.h

@@ -36,7 +36,21 @@
  * The station is still assumed to belong to the AP interface it was added
  * to.
  *
- * TODO: need more info?
+ * Station handling varies per interface type and depending on the driver's
+ * capabilities.
+ *
+ * For drivers supporting TDLS with external setup (WIPHY_FLAG_SUPPORTS_TDLS
+ * and WIPHY_FLAG_TDLS_EXTERNAL_SETUP), the station lifetime is as follows:
+ *  - a setup station entry is added, not yet authorized, without any rate
+ *    or capability information, this just exists to avoid race conditions
+ *  - when the TDLS setup is done, a single NL80211_CMD_SET_STATION is valid
+ *    to add rate and capability information to the station and at the same
+ *    time mark it authorized.
+ *  - %NL80211_TDLS_ENABLE_LINK is then used
+ *  - after this, the only valid operation is to remove it by tearing down
+ *    the TDLS link (%NL80211_TDLS_DISABLE_LINK)
+ *
+ * TODO: need more info for other interface types
  */
 
 /**