net/mlx5e: Use partial batches in legacy RQ

Legacy RQ allocates WQEs in batches. If the batch allocation fails, the
pages of the allocated part are released. This commit changes this
behavior to allow to use the pages that have been already allocated.

After this change, we need to be careful about indexing rq->wqe.frags[].
The WQ size is a power of two that divides by wqe_bulk (8), and the old
code used whole bulks, which allowed to use indices [8*K; 8*K+7] without
overflowing. Now that the bulks may be partial, the range can start at
any location (not only at 8*K), so we need to wrap them around to avoid
out-of-bounds array access.

Signed-off-by: Maxim Mikityanskiy <maximmi@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>