batman-adv: Add throughput_override hardif genl configuration

The B.A.T.M.A.N. V implementation tries to estimate the link throughput of
an interface to an originator using different automatic methods. It is
still possible to overwrite it the link throughput for all reachable
originators via this interface.

The BATADV_CMD_SET_HARDIF/BATADV_CMD_GET_HARDIF commands allow to set/get
the configuration of this feature using the u32
BATADV_ATTR_THROUGHPUT_OVERRIDE attribute. The used unit is in 100 Kbit/s.
If the value is set to 0 then batman-adv will try to estimate the
throughput by itself.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>