bgpd: efficient NLRI packing for AFs != ipv4-unicast

bgpd: efficient NLRI packing for AFs != ipv4-unicast

ISSUE:

  Currently, for non-ipv4-unicast address families where prefixes are
  encoded in MP_REACH/MP_UNREACH attributes, BGP ends up sending one
  prefix per UPDATE message. This is quite inefficient. The patch
  addresses the issue.

PATCH:

  We introduce a scratch buffer in the peer structure that stores the
  MP_REACH/MP_UNREACH attributes for non-ipv4-unicast families. This
  enables us to encode multiple prefixes. In the end, the two buffers
  are merged to create the UPDATE packet.

Signed-off-by: Pradosh Mohapatra <pmohapat@cumulusnetworks.com>
Reviewed-by: Daniel Walton <dwalton@cumulusnetworks.com>
[DL: removed no longer existing bgp_packet_withdraw prototype]
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>