IB/umad: Avoid destroying device while it is accessed

ib_umad_reg_agent2() and ib_umad_reg_agent() access the device name in
dev_notice(), while concurrently, ib_umad_kill_port() can destroy the
device using device_destroy().

        cpu-0                               cpu-1
        -----                               -----
    ib_umad_ioctl()
        [...]                            ib_umad_kill_port()
                                              device_destroy(dev)

        ib_umad_reg_agent()
            dev_notice(dev)

Therefore, first mark ib_dev as NULL, to block any further access in file
ops, unregister the mad agent and destroy the device at the end after
mutex is unlocked.

This ensures that device doesn't get destroyed, while it may get accessed.

Fixes: 0f29b46d ("IB/mad: add new ioctl to ABI to support new registration options")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Jack Morgenstein <jackm@mellanox.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>