samples/bpf: Add missing option to xdp_fwd usage

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

samples/bpf/xdp_fwd_user.c

@@ -67,6 +67,8 @@ static void usage(const char *prog)
 		"usage: %s [OPTS] interface-list\n"
 		"\nOPTS:\n"
 		"    -d    detach program\n"
+		"    -S    use skb-mode\n"
+		"    -F    force loading prog\n"
 		"    -D    direct table lookups (skip fib rules)\n",
 		prog);
 }