bonding: use IS_ERR instead of NULL check in bond_create_debugfs

Because debugfs_create_dir returns ERR_PTR, so IS_ERR should be used to
check whether the directory is successfully created.

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>