bgpd: fix bgp vrf instance creation from implicit

bgpd: fix bgp vrf instance creation from implicit

In bgp route leak, when import vrf x is executed,
it creates bgp instance as hidden with asn value as unspecified.

When router bgp x is configured ensure the correct as,
asnotation is applied otherwise running config shows asn value as 0.

This can lead to frr-reload failure when any FRR config change.

Fix:
Move asn and asnotiation, as_pretty value in common done section,
so when bgp_create gets existing instance but before returning
update asn and required fields in common section.

In bgp_create(): when returning for hidden at least update asn
and required when bgp instance created implicitly due to vrf leak.

if (hidden) {
    bgp = bgp_old;
    goto peer_init; <<<
}

Before fix:
show running:

router bgp 0 vrf purple
 bgp router-id 10.10.3.11
 !
 address-family ipv4 unicast
  redistribute static
  import vrf blue
 exit-address-family
 !
 address-family ipv6 unicast
  import vrf blue
 exit-address-family
 !
 address-family l2vpn evpn
  advertise ipv4 unicast
  advertise ipv6 unicast
 exit-address-family
exit

Testing:

1) following snippet config:
router bgp 63420 vrf blue
 import vrf purple
router bgp 63420 vrf purple
 import vrf blue
2) restart frr leads to the running config with 0 asn value.

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