[IPV4]: Correct rp_filter help text.

As mentioned in http://bugzilla.kernel.org/show_bug.cgi?id=5015


The helptext implies that this is on by default.
This may be true on some distros (Fedora/RHEL have it enabled
in /etc/sysctl.conf), but the kernel defaults to it off.

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>