net/mlx5e: Add padding when needed in UMR WQEs

Per the device spec, MTTs/KLMs list in a UMR WQE must be aligned to 64B.
Per our SW design, the MTT/KLMs list would need alignment only if it's
too small, for example on PPC when PAGE_SIZE is 64KB, and only 4 pages
are needed to cover a MPWQE of size 256KB.

Padding, if needed, is taken into account when calculating the UMR WQE
fields (ds_cnt and xlt_octowords), however no entries are provided,
instead garbage is passed.

No real harm though, as these parts act as gaps between the RX MPWQEs
and not used by any of them. Hence, in practice, device does not try to
write any incoming packet to them. Still, prefer providing clean padding
marking the end of the list, and do not map garbage into the RQ memory
region.

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