bgpd: Fix params/checks for route leaking and L3VPN

bgpd: Fix params/checks for route leaking and L3VPN

When routes are injected into the VPN table and then leaked into candidate
VRFs, the source should be the default instance. Also, the loop check when
withdrawing routes from a VRF should be that the route's origin isn't that
VRF; this handles VRF route leaking also and is consistent with checks in
other places.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
(cherry picked from commit 0149d2848c59bfb3277592caf0a5d5e07a2de872)

Ticket: CM-20256