bgpd: Allow EVPN advertise route-map to modify attributes

bgpd: Allow EVPN advertise route-map to modify attributes

Ensure that the EVPN advertise route-map is applied on a copy of the
original path_info and associated attribute, so that if the route-map
has SET clauses, they can operate properly. This closely follows
the model already in use in other route-map application code.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Don Slice <dslice@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>