bgpd: fix empty advertised-routes for 2-tier safis

bgpd: fix empty advertised-routes for 2-tier safis

'show bgp ipv[46] vpn neighbors ... advertised-routes' was displaying
empty output due to new command syntax using show_adj_routes() which
assumed each bgp_table was single-tier (not nested).  This fixes that
assumption for safis with a two-tier bgp_table (SAFI_MPLS_VPN,
SAFI_ENCAP, and SAFI_EVPN).

Before:
ub18# show bgp ipv6 vpn neighbors 2001:db8:cafe::2 advertised-routes
ub18#

After:
ub20# show bgp ipv6 vpn neighbors 2001:db8:cafe::1 advertised-routes
BGP table version is 2, local router ID is 100.64.0.222, vrf id 0
Default local pref 100, local AS 1
Status codes:  s suppressed, d damped, h history, * valid, > best, = multipath,
               i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes:  i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
Route Distinguisher: 30:30
*> 2::2/128         ::                       0    100  32768 i
*> 2::22/128        ::                       0    100  32768 i
Route Distinguisher: 33:33
*> 2::2/128         ::                       0    100  32768 i
*> 2::22/128        ::                       0    100  32768 i

Total number of prefixes 4

Signed-off-by: Trey Aspelund <taspelund@nvidia.com>