ipv4: Fix rp_filter description in net/ipv4/Kconfig.

The reverse path filter (rp_filter) will NOT get enabled
when enabling forwarding.  Read the code and tested in
in practice.

Most distributions do enable it in startup scripts.

Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>