pimd - Fix rpf lookup to what it was originally

pimd - Fix rpf lookup to what it was originally

I tried to be smart and skirt around rpf lookup if I knew
the incoming interface.  This turns out to be not necessarily
a good thing because we can easily have asymetrical routing.

This fix removes the attempt to cache the ifp we received
the incoming packet on and just lets the lookup work like
it should.

Additionally it removes the weird hardcoding of the rpf
interface from the register stuff.

Ticket: CM-12530
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
reb