IB/ucm: utilize new cdev_device_add helper function

The use after free is not triggerable here because the cdev holds
the module lock and the only device_unregister is only triggered by
module unload, however make the change for consistency.

To make this work the cdev_del needs to move out of the struct device
release function.

This cleans up the error path significantly and thus also fixes a minor
bug where the devnum would not be released if cdev_add failed.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>