bgpd: fix evpn ecommunity auto rts

bgpd: fix evpn ecommunity auto rts

Evpn extended communities like auto rts (import/export) should
check if its present in list before adding it, to avoid duplicate
addition.
L3vni_add callback from zebra to bgp may see updates to vnis.
The auto import/export rt derivation may call multiple times.

Testing Done:

Before:
TORC11# show bgp l2vpn evpn vni 4001
VNI: 4001 (known to the kernel)
  Type: L3
  Tenant VRF: vrf1
  RD: 45.0.2.2:3
  ...
  Import Route Target:
    5546:4001
    5546:4001
  Export Route Target:
    5546:4001
    5546:4001

After:
VNI: 4001 (known to the kernel)
  Type: L3
  Tenant VRF: vrf1
  RD: 45.0.2.2:3
  ...
 Import Route Target:
    5546:4001
  Export Route Target:
    5546:4001

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