bgpd: fix flushing ipv6 flowspec entries when peering stops

bgpd: fix flushing ipv6 flowspec entries when peering stops

When a BGP flowspec peering stops, the BGP RIB entries for IPv6
flowspec entries are removed, but not the ZEBRA RIB IPv6 entries.

Actually, when calling bgp_zebra_withdraw() function call, only
the AFI_IP parameter is passed to the bgp_pbr_update_entry() function
in charge of the Flowspec add/delete in zebra. Fix this by passing
the AFI parameter to the bgp_zebra_withdraw() function.

Note that using topotest does not show up the problem as the
flowspec driver code is not present and was refused. Without that,
routes are not installed, and can not be uninstalled.

Fixes: 529efa234655 ("bgpd: allow flowspec entries to be announced to zebra")
Link: https://github.com/FRRouting/frr/pull/2025
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>