rds: tcp: cancel all worker threads before shutting down socket

rds: tcp: cancel all worker threads before shutting down socket

We could end up executing rds_conn_shutdown before the rds_recv_worker
thread, then rds_conn_shutdown -> rds_tcp_conn_shutdown can do a
sock_release and set sock->sk to null, which may interleave in bad
ways with rds_recv_worker, e.g., it could result in:

    "BUG: unable to handle kernel NULL pointer dereference at 0000000000000078"
       [ffff881769f6fd70] release_sock at ffffffff815f337b
       [ffff881769f6fd90] rds_tcp_recv at ffffffffa043c888 [rds_tcp]
       [ffff881769f6fdb0] rds_recv_worker at ffffffffa04a4810 [rds]
       [ffff881769f6fde0] process_one_work at ffffffff810a14c1
       [ffff881769f6fe40] worker_thread at ffffffff810a1940
       [ffff881769f6fec0] kthread at ffffffff810a6b1e

cancel send/recv worker threads before shutting down connection.

Also, do not enqueue any new shutdown workq items when the connection is
shutting down (this may happen for rds-tcp in softirq mode, if a FIN
or CLOSE is received while the modules is in the middle of an unload)

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Wei Lin Guay <wei.lin.guay@oracle.com>
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>
(cherry picked from commit UEK4 bf1f0c8c007c58eae14622c6f9a3fe36f6b19d5a)

Orabug: 28298156

Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>