RDMA/cma: Fix use after destroy access to net namespace for IPoIB

There are few issues with validation of netdevice and listen id lookup
for IB (IPoIB) while processing incoming CM request as below.

1. While performing lookup of bind_list in cma_ps_find(), net namespace
of the netdevice can get deleted in cma_exit_net(), resulting in use
after free access of idr and/or net namespace structures.
This lookup occurs from the workqueue context (and not userspace
context where net namespace is always valid).

           CPU0                              CPU1
           ====                              ====

 bind_list = cma_ps_find();
                                     move netdevice to new namespace
                                     delete net namespace
                                        cma_exit_net()
                                           idr_destroy(idr);

 [..]
 cma_find_listener(bind_list, ..);

2. While netdevice is validated for IP address in given net namespace,
netdevice's net namespace and/or ifindex can change in
cma_get_net_dev() and cma_match_net_dev().

Above issues are overcome by using rcu lock along with netdevice
UP/DOWN state as described below.
When a net namespace is getting deleted, netdevice is closed and
shutdown before moving it back to init_net namespace.
change_net_namespace() synchronizes with any existing use of netdevice
before changing the netdev properties such as net or ifindex.
Once netdevice IFF_UP flags is cleared, such fields are not guaranteed
to be valid.
Therefore, rcu lock along with netdevice state check ensures that,
while route lookup and cm_id lookup is in progress, netdevice of
interest won't migrate to any other net namespace.
This ensures that associated net namespace of netdevice won't get
deleted while rcu lock is held for netdevice which is in IFF_UP state.

Fixes: fa20105e ("IB/cma: Add support for network namespaces")
Fixes: 4be74b42 ("IB/cma: Separate port allocation to network namespaces")
Fixes: f887f2ac

 ("IB/cma: Validate routing of incoming requests")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>