cfg80211: fix duplicated scan entries after channel switch

cfg80211: fix duplicated scan entries after channel switch

When associated BSS completes channel switch procedure, its channel
record needs to be updated. The existing mac80211 solution was
extended to cfg80211 in commit 5dc8cdce1d72 ("mac80211/cfg80211:
update bss channel on channel switch").

However that solution still appears to be incomplete as it may lead
to duplicated scan entries for associated BSS after channel switch.
The root cause of the problem is as follows. Each BSS entry is
included into the following data structures:
- bss list rdev->bss_list
- bss search tree rdev->bss_tree
Updating BSS channel record without rebuilding bss_tree may break
tree search since cmp_bss considers all of the following: channel,
bssid, ssid. When BSS channel is updated, but its location in bss_tree
is not updated, then subsequent search operations may fail to locate
this BSS since they will be traversing bss_tree in wrong direction.
As a result, for scan performed after associated BSS channel switch,
cfg80211_bss_update may add the second entry for the same BSS to both
bss_list and bss_tree, rather then update the existing one.

To summarize, if BSS channel needs to be updated, then bss_tree should
be rebuilt in order to put updated BSS entry into a proper location.

This commit suggests the following straightforward solution:
- if new entry has been already created for BSS after channel switch,
  then use its IEs to update known BSS entry and then remove new
  entry completely
- use rb_erase/rb_insert_bss reinstall updated BSS in bss_tree
- for nontransmit BSS entry, the whole transmit BSS hierarchy
  is updated

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Link: https://lore.kernel.org/r/20190726163922.27509-3-sergey.matyukevich.os@quantenna.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>