bpf: make mlx4 compatible w/ bpf_xdp_adjust_tail

w/ bpf_xdp_adjust_tail helper xdp's data_end pointer could be changed as
well (only "decrease" of pointer's location is going to be supported).
changing of this pointer will change packet's size.
for mlx4 driver we will just calculate packet's length unconditionally
(the same way as it's already being done in mlx5)
Acked-by: NAlexei Starovoitov <ast@kernel.org>
Acked-by: NTariq Toukan <tariqt@mellanox.com>
Signed-off-by: NNikita V. Shirokov <tehnerd@tehnerd.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

drivers/net/ethernet/mellanox/mlx4/en_rx.c

@@ -775,8 +775,8 @@ int mlx4_en_process_rx_cq(struct net_device *dev, struct mlx4_en_cq *cq, int bud
 
 			act = bpf_prog_run_xdp(xdp_prog, &xdp);
 
+			length = xdp.data_end - xdp.data;
 			if (xdp.data != orig_data) {
-				length = xdp.data_end - xdp.data;
 				frags[0].page_offset = xdp.data -
 					xdp.data_hard_start;
 				va = xdp.data;