cfg80211: VLAN offload support for set_key and set_sta_vlan

This provides an alternative mechanism for AP VLAN support where a
single netdev is used with VLAN tagged frames instead of separate
netdevs for each VLAN without tagged frames from the WLAN driver.

By setting NL80211_EXT_FEATURE_VLAN_OFFLOAD flag the driver indicates
support for a single netdev with VLAN tagged frames. Separate
VLAN-specific netdevs can be added using RTM_NEWLINK/IFLA_VLAN_ID
similarly to Ethernet. NL80211_CMD_NEW_KEY (for group keys),
NL80211_CMD_NEW_STATION, and NL80211_CMD_SET_STATION will optionally
specify vlan_id using NL80211_ATTR_VLAN_ID.

Signed-off-by: Gurumoorthi Gnanasambandhan <gguru@codeaurora.org>
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Link: https://lore.kernel.org/r/20191031214640.5012-1-jouni@codeaurora.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>