samples/bpf: Add support for SKB_MODE to xdp1 and xdp_tx_iptunnel

Add option to xdp1 and xdp_tx_iptunnel to insert xdp program in
SKB_MODE:
 - update set_link_xdp_fd to take a flags argument that is added to the
   RTM_SETLINK message

 - Add -S option to xdp1 and xdp_tx_iptunnel user code. When passed in
   XDP_FLAGS_SKB_MODE is set in the flags arg passed to set_link_xdp_fd

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>