net/mlx5e: Use fragments of the same size in non-linear legacy RQ with XDP

XDP multi buffer implementation in the kernel assumes that all fragments
have the same size. bpf_xdp_frags_increase_tail uses this assumption to
get the size of the last fragment, and __xdp_build_skb_from_frame uses
it to calculate truesize as nr_frags * xdpf->frame_sz.

The current implementation of mlx5e uses fragments of different size in
non-linear legacy RQ. Specifically, the last fragment can be larger than
the others. It's an optimization for packets smaller than MTU.

This commit adapts mlx5e to the kernel limitations and makes it use
fragments of the same size, in order to add support for XDP multi
buffer. The change is applied only if XDP is active, otherwise the old
optimization still applies.

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