bgpd: backpressure - fix to properly remove dest for bgp under deletion

bgpd: backpressure - fix to properly remove dest for bgp under deletion

In case of imported routes (L3vni/vrf leaks), when a bgp instance is
being deleted, the peer->bgp comparision with the incoming bgp to remove
the dest from the pending fifo is wrong. This can lead to the fifo
having stale entries resulting in crash.

Two changes are done here.
 - Instead of pop/push items in list if the struct bgp doesnt match,
   simply iterate the list and remove the expected ones.

 - Corrected the way bgp is fetched from dest rather than relying on
   path_info->peer so that it works for all kinds of routes.

Ticket :#3980988

Signed-off-by: Chirag Shah <chirag @nvidia.com>
Signed-off-by: Rajasekar Raja <rajasekarr@nvidia.com>