RDMA/core: Introduce ib_core_device to hold device

In order to support sysfs entries in multiple net namespaces for a rdma
device, introduce a ib_core_device whose scope is limited to hold core
device and per port sysfs related entries.

This is preparation patch so that multiple ib_core_devices in each net
namespace can be created in subsequent patch who all can share ib_device.

(a) Move sysfs specific fields to ib_core_device.
(b) Make sysfs and device life cycle related routines to work on
    ib_core_device.
(c) Introduce and use rdma_init_coredev() helper to initialize
    coredev fields.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>