pim-nexthop: set the correct nexthop address in the rpf info.

pim-nexthop: set the correct nexthop address in the rpf info.

When a nexthop lookup is done we can get an ECMP output. But not all
nexthops are pim neighbors. If for this reason PIM chose a nexthop other
than the first the rpf info was not being set correctly i.e.
nexthop ip was still the one associated with the first path but
interface was set to the one associated with second path.

This problem is seen on a link flap in the CLOS topology.

Ticket: CM-13714
Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>
Acked-by: Donald Sharp <sharpd@cumulusnetworks.com>