[NET]: Cleanup calling netdev notifiers.

The call_netdev_notifiers routine can successfully be used in
the net/core_dev.c itself.

This will save 6 lines of code and 62 ;) bytes of .text section.

62 is rather small, but I have one more patch saving ~30 bytes
from netns code (sent to Eric), so altogether they can save
some more noticeable amount.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>