rds: flush and destroy workqueue rds_aux_wq and fix creation order.

rds: flush and destroy workqueue rds_aux_wq and fix creation order.

in commit f05d77d46d172127d3f96538a62764a2a589a61b

    rds: Add Automatic Path Migration support

    RDS APM supports automatic connection failover in case of path
    failure, and connection failback when the path recovers.

    RDS APM is enabled by module parameter rds_ib_enable_apm (disabled by
    default).

workqueue rds_aux_wq is not destroyed and it should be create prior to
rds_trans_register since rds_trans_register callbacks can use rds_aux_wq.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>