RDMA/core: Implement compat device/sysfs tree in net namespace

Implement compatibility layer sysfs entries of ib_core so that non
init_net net namespaces can also discover rdma devices.

Each non init_net net namespace has ib_core_device created in it.
Such ib_core_device sysfs tree resembles rdma devices found in
init_net namespace.

This allows discovering rdma devices in multiple non init_net net
namespaces via sysfs entries and helpful to rdma-core userspace.
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

include/rdma/ib_verbs.h

@@ -2559,6 +2559,7 @@ struct ib_core_device {
 	 * union of ib_core_device and device exists in ib_device.
 	 */
 	struct device dev;
+	possible_net_t rdma_net;
 	struct kobject *ports_kobj;
 	struct list_head port_list;
 	struct ib_device *owner; /* reach back to owner ib_device */
@@ -2636,6 +2637,11 @@ struct ib_device {
 	struct work_struct unregistration_work;
 
 	const struct rdma_link_ops *link_ops;
+
+	/* Protects compat_devs xarray modifications */
+	struct mutex compat_devs_mutex;
+	/* Maintains compat devices for each net namespace */
+	struct xarray compat_devs;
 };
 
 struct ib_client {