net: call notifiers for mtu change even if iface is not up

Do the same thing as in set mac. Call notifiers every time.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>