net/mlx5e: Keep a separate MKey for striding RQ

Currently, rq->mkey_be keeps a big-endian value of either the PA MKey
(for legacy RQ, no address translation) or MTT MKey (for striding RQ,
direct address translation). Striding RQ stores the same value in
rq->umr_mkey in the native endianness.

The next commit will make striding RQ use KSM MKey (indirect address
translation) for the unaligned mode of XSK, which will require storing
both KSM MKey and PA MKey in the RQ struct. This commit optimizes fields
of mlx5e_rq: umr_mkey is removed (it's redundant), mkey_be always points
to the PA MKey, and mpwqe.umr_mkey_be points to the MTT MKey (or to the
KSM MKey, starting from the next commit).

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>