bgpd: Clean extended communities for VRF routes imported from EVPN

bgpd: Clean extended communities for VRF routes imported from EVPN

According to draft-ietf-bess-evpn-ipvpn-interworking-13, strip
the following extended communities for VRF routes imported from EVPN.

  a. BGP Encapsulation extended communities.
  b. Route Target extended communities.
  c. All the extended communities of type EVPN.

As the corresponding fields will not be always covered by extended
communities, add them to attr hash key directly.

Signed-off-by: Xiao Liang <shaw.leon@gmail.com>