If a dedicated (off-channel) radar detection hardware (chain)
is available in the hardware/driver, allow this to be used by
calling the NL80211_CMD_RADAR_DETECT command with a new flag
attribute requesting off-channel radar detection is used.

Offchannel CAC (channel availability check) avoids the CAC
downtime when switching to a radar channel or when turning on
the AP.

Drivers advertise support for this using the new feature flag
NL80211_EXT_FEATURE_RADAR_OFFCHAN.
Tested-by: Evelyn Tsai <evelyn.tsai@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/7468e291ef5d05d692c1738d25b8f778d8ea5c3f.1634979655.git.lorenzo@kernel.org
Link: https://lore.kernel.org/r/1e60e60fef00e14401adae81c3d49f3e5f307537.1634979655.git.lorenzo@kernel.org
Link: https://lore.kernel.org/r/85fa50f57fc3adb2934c8d9ca0be30394de6b7e8.1634979655.git.lorenzo@kernel.org
Link: https://lore.kernel.org/r/4b6c08671ad59aae0ac46fc94c02f31b1610eb72.1634979655.git.lorenzo@kernel.org
Link: https://lore.kernel.org/r/241849ccaf2c228873c6f8495bf87b19159ba458.1634979655.git.lorenzo@kernel.org
[remove offchan_mutex, fix cfg80211_stop_offchan_radar_detection(),
 remove gfp_t argument, fix documentation, fix tracing]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>