netdev-linux: Cache flags using netlink.

netdev-linux: Cache flags using netlink.

Before this patch, every request for a 'netdev_dev''s flags
required an ioctl call.  This occurred every time
netdev_get_carrier() was called, which theoretically was very often
if there were a large number of devices.  We were already using
netlink to keep track of the IFF_RUNNING flag. This patch
generalizes the code to keep track of all flags using the same
netlink code.

Signed-off-by: Ethan Jackson <ethan@nicira.com>