[NET]: Fix sysctl net.core.dev_weight

Changing the sysctl net.core.dev_weight has no effect because the weight
of the backlog devices is set during initialization and never changed.

This patch propagates any changes to the global value affected by sysctl
to the per-cpu devices. It is done every time the packet handler
function is run.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>