pimd: prevent LHR from register forwarding packets for non-FHR sources

pimd: prevent LHR from register forwarding packets for non-FHR sources

SPT switchover handling is done by adding pimreg in the OIL of the (*, G)
entry on the LHR. This causes multicast data with that destination to be
sent to pimd as IGMPMSG_WHOLEPKT. These packets trigger creation of (S,G)
and also register forwarding. However register forwarding must only be done
if the router is also a FHR. That FHR check was missing causing strange
source registrations from multicast routers that were not directly
connected to the source.

Relevant logs from LHR -
PIM: pim_mroute_msg: pim kernel upcall WHOLEPKT type=3 ip_p=0 from fd=9 for (S,G)=(6.0.0.30,239.1.1.111) on pimreg vifi=0  size=98
PIM: Sending (6.0.0.30,239.1.1.111) Register Packet to 81.0.0.5
PIM: pim_register_send: Sending (6.0.0.30,239.1.1.111) Register Packet to 81.0.0.5 on swp2

And 6.0.0.30 is clearly not directly connected on that router -
root@tor-11:~# ip route |grep 6.0.0.30 -A2
6.0.0.30 proto ospf metric 20
	nexthop via 6.0.0.22 dev swp1 weight 1 onlink
	nexthop via 6.0.0.23 dev swp2 weight 1 onlink
root@tor-11:~#

Ticket: CM-24549

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>