Doc: Refer to ip-sysctl.txt for strict vs. loose rp_filter mode

The IP_ADVANCED_ROUTER Kconfig describes the rp_filter
proc option.  Recent changes added a loose mode.
Instead of documenting this change too places, refer to
the document describing it:
 Documentation/networking/ip-sysctl.txt

I'm considering moving the rp_filter description away
from the Kconfig file into ip-sysctl.txt.

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