bgpd: Free up changes to attr that the speculative route-map applied

bgpd: Free up changes to attr that the speculative route-map applied

So we have the ability to apply speculative route-maps to
neighbor display to see what the changes would look like
via some show commands.  When we do this we make a
shallow copy of the attr data structure and then pass
it around for applying the routemap.  After we've applied
this route-map and displayed it we really need to clean
up memory that the route-map application applied.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>