net/mlx5: LAG, replace pf array with xarray

net/mlx5: LAG, replace pf array with xarray

Replace the fixed-size array with a dynamic xarray.

This commit changes:
- Adds mlx5_lag_pf() helper for consistent xarray access
- Converts all direct pf[] accesses to use the new helper/macro
- Dynamically allocates lag_func entries via xa_store/xa_load

No functional changes intended. This prepares the LAG infrastructure
for future flexibility in device indexing.

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/20260309093435.1850724-4-tariqt@nvidia.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>