net/mlx5e: Restrict the combination of large MTU and XDP

Add checks in control path upon an MTU change or an XDP program set,
to prevent reaching cases where large MTU and XDP are set simultaneously.

This is to make sure we allow XDP only with the linear RX memory scheme,
i.e. a received packet is not scattered to different pages.
Change mlx5e_rx_get_linear_frag_sz() accordingly, so that we make sure
the XDP configuration can really be set, instead of assuming that it is.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>