IB: Allow userspace to set node description

Expose a writable "node_desc" sysfs attribute for InfiniBand devices.
This allows userspace to update the node description with information
such as the node's hostname, so that IB network management software
can tie its view to the real world.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>