cfg80211/mac80211: add mesh_param "mesh_nolearn" to skip path discovery

cfg80211/mac80211: add mesh_param "mesh_nolearn" to skip path discovery

Currently, before being able to forward a packet between two 802.11s
nodes, both a PLINK handshake is performed upon receiving a beacon and
then later a PREQ/PREP exchange for path discovery is performed on
demand upon receiving a data frame to forward.

When running a mesh protocol on top of an 802.11s interface, like
batman-adv, we do not need the multi-hop mesh routing capabilities of
802.11s and usually set mesh_fwding=0. However, even with mesh_fwding=0
the PREQ/PREP path discovery is still performed on demand. Even though
in this scenario the next hop PREQ/PREP will determine is always the
direct 11s neighbor node.

The new mesh_nolearn parameter allows to skip the PREQ/PREP exchange in
this scenario, leading to a reduced delay, reduced packet buffering and
simplifies HWMP in general.

mesh_nolearn is still rather conservative in that if the packet destination
is not a direct 11s neighbor, it will fall back to PREQ/PREP path
discovery.

For normal, multi-hop 802.11s mesh routing it is usually not advisable
to enable mesh_nolearn as a transmission to a direct but distant neighbor
might be worse than reaching that same node via a more robust /
higher throughput etc. multi-hop path.

Cc: Sven Eckelmann <sven@narfation.org>
Cc: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Linus Lüssing <ll@simonwunderlich.de>
Link: https://lore.kernel.org/r/20200617073034.26149-1-linus.luessing@c0d3.blue
[fix nl80211 policy to range 0/1 only]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>