bgpd: Fix bgpd doing vpn_leak_postchange_all() every time "router bgp ASNUM" command is entered in vtysh
In rare cases when the default BGP instance is instantiated after VRF bgp instances (see comment to bgp_mplsvpn.c:vpn_leak_postchange_all() for an example), the "router bgp" command needs to call vpn_leak_postchange_all() to start the route leaking process. The issue was it was never checked if the "router bgp" command was used to create the default BGP instance or just to enter into "router bgp" command context. This resulted in vpn_leak_postchange_all() executed every time (and vpn routes re-announced to all peers) when the user was entering "router bgp" command context.
Signed-off-by: Anton Degtyarev <anton@cumulusnetworks.com>