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

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.

Fixes: 7f46a0b7327a ("net/mlx5: Lag, add debugfs to query hardware lag state")
Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20260330194015.53585-2-tariqt@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>