net: add busy_poll device feature

Currently there is no way how to find out if a device supports busy
polling. So add a feature and make it dependent on ndo_busy_poll
existence.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/linux/netdev_features.h
Simple merge

diff --cc net/core/dev.c
Simple merge

diff --cc net/core/ethtool.c
Simple merge