RDS: Update rds_conn_destroy to be MP capable

Refactor rds_conn_destroy() so that the per-path dismantling
is done in rds_conn_path_destroy, and then iterate as needed
over rds_conn_path_destroy().

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>