net/mlx5: Use generic definition for UMR KLM alignment

MLX5_UMR_KLM_ALIGNMENT is in units of number of entries, while
MLX5_UMR_MTT_ALIGNMENT (generalized and renamed to
MLX5_UMR_FLEX_ALIGNMENT) is in byte units. This is misleading and
confusing.
Replace this KLM definition with one based on the generic definition.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>