BGP: Only accept prefixes for negotiated address families

BGP: Only accept prefixes for negotiated address families

When handling a received Update message, only process and store the
prefixes if the corresponding address family has been negotiated with
the peer. Prior to this change, the receive processing only checked
whether the address family was locally configured, trusting to the peer
to not advertise prefixes for an address family that has not been
negotiated. Most implementations conform to this but a misbehavior could
result in processing and memory overhead.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Daniel Walton <dwalton@cumulusnetworks.com>
Ticket: CM-5594
Reviewed By: CCR-3946
Testing Done: Sanity test (good case)