bgpd: authorise to select bgp self peer prefix on rr case

bgpd: authorise to select bgp self peer prefix on rr case

This commit addresses an issue that happens when using bgp
peering with a rr client, with a received prefix which is the
local ip address of the bgp session.

When using bgp ipv4 unicast session, the local prefix is
received by a peer, and finds out that the proposed prefix
and its next-hop are the same. To avoid a route loop locally,
no nexthop entry is referenced for that prefix, and the route
will not be selected.

When the received peer is a route reflector, the prefix has
to be selected, even if the route can not be installed locally.

Fixes: ("fb8ae704615c") bgpd: prevent routes loop through itself
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>