[NET]: Don't declare extern variables in net/core/sysctl_net_core.c

Some are already declared in include/linux/netdevice.h, while
some others (xfrm ones) need to be declared.

The driver/net/rrunner.c just uses same extern as well, so
cleanup it also.

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