ipv4: route: restore skb_dst_set in inet_rtm_getroute

recent updates to inet_rtm_getroute dropped skb_dst_set in
inet_rtm_getroute. This patch restores it because it is
needed to release the dst correctly.

Fixes: 3765d35e ("net: ipv4: Convert inet_rtm_getroute to rcu versions of route lookup")
Reported-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>