mac80211: code for on-demand Hybrid Wireless Mesh Protocol

This file implements the on-demand Hybrid Wireless Mesh Protocol, at this moment
using hop-count as the metric. When no mesh path exists for a given destination
or the mesh path is not active, frames addressed to that destination will be
queued and a Path Request frame will be sent. Queued frames will be sent when
the path is resolved (usually after reception of a Path Response) or discarded
if discovery times out. Path Requests will also be sent to refresh paths that
are being used and are close to expiring.

Path Errors are sent when a path discovery process triggered by the attempt to
forward a frame originated in a different mesh point times out. Path Errors are
also sent when a peer link is determined to be unreachable because of high error
rates.

Multiple destination support in Path Requests and Path Errors and precursors
have not been implemented yet.

Signed-off-by: Luis Carlos Cobo <luisca@cozybit.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>