net/mlx5: lag: Check for LAG device before creating debugfs

In the Linux kernel, the following vulnerability has been resolved:

net/mlx5: lag: Check for LAG device before creating debugfs

__mlx5_lag_dev_add_mdev() may return 0 (success) even when an error

occurs that is handled gracefully. Consequently, the initialization

flow proceeds to call mlx5_ldev_add_debugfs() even when there is no

valid LAG context.

mlx5_ldev_add_debugfs() blindly created the debugfs directory and

attributes. This exposed interfaces (like the members file) that rely on

a valid ldev pointer, leading to potential NULL pointer dereferences if

accessed when ldev is NULL.

Add a check to verify that mlx5_lag_dev(dev) returns a valid pointer

before attempting to create the debugfs entries.