bgpd: Process directly connected IBGP peers upon interface down

bgpd: Process directly connected IBGP peers upon interface down

When we have a single-hop BFD session for any peering, it really means
that the peering is directly connected (maybe over a L2 network), whether
it is IBGP or EBGP. In such a case, upon link down, immediately process
IBGP peers too (and bring them down), not just EBGP peers.

This change eliminates some peculiar state transitions in specific IBGP
topologies, thus getting rid of the problem of nexthops remaining inactive
in the zebra RIB.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Daniel Walton <dwalton@cumulusnetworks.com>
Ticket: CM-12390
Reviewed By: CCR-5156
Testing Done: Manual, bgp-smoke