mlx4: Add XDP frame size and adjust max XDP MTU

The mlx4 drivers size of memory backing the RX packet is stored in
frag_stride. For XDP mode this will be PAGE_SIZE (normally 4096).
For normal mode frag_stride is 2048.

Also adjust MLX4_EN_MAX_XDP_MTU to take tailroom into account.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Cc: Saeed Mahameed <saeedm@mellanox.com>
Link: https://lore.kernel.org/bpf/158945341893.97035.2688142527052329942.stgit@firesoul