BGP: Check in multipath comparison before invoking sockunion_cmp

BGP: Check in multipath comparison before invoking sockunion_cmp

During route selection for Multipath routes, when multiple peers are
flapping, it is possible that the old (former) multipath list of routes
for a destination may include routes from peers which are no longer in
Established state. When the new multipath list is compared against the
old list to identify changes, additional checks are needed to avoid
comparing connection addresses if the peer is not in Established state.
This patch introduces those checks.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Daniel Walton <dwalton@cumulusnetworks.com>
Reviewed-by:   Don Slice <dslice@cumulusnetworks.com>
Ticket: CM-9671
Reviewed By: CCR-4277
Testing Done: Manual (problem could not be replicated to verify)