bgpd: fix the add-path code to understand the mpls-vpn safi

bgpd: fix the add-path code to understand the mpls-vpn safi

In order to iterate over MPLS VPN routes, it's necessary to use
two nested loops (the outer loop iterates over the MPLS VPN RDs,
and the inner loop iterates over the VPN routes from that RD).

The add-path code wasn't doing this, which was leading to lots of
crashes when add-path was enabled for the MPLS VPN SAFI. This patch
fixes the problem.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>