xdp_fwd usage() is missing the introduction of the "-S"
and "-F" options, this patch adds it.

Fixes: d50ecc46 ("samples/bpf: Attach XDP programs in driver mode by default")
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Link: https://lore.kernel.org/bpf/20210615135554.29158-1-wanghai38@huawei.com