samples/bpf: fix XDP_FLAGS_SKB_MODE detach for xdp_tx_iptunnel

samples/bpf: fix XDP_FLAGS_SKB_MODE detach for xdp_tx_iptunnel

The xdp_tx_iptunnel program can be terminated in two ways, after
N-seconds or via Ctrl-C SIGINT.  The SIGINT code path does not
handle detatching the correct XDP program, in-case the program
was attached with XDP_FLAGS_SKB_MODE.

Fix this by storing the XDP flags as a global variable, which is
available for the SIGINT handler function.

Fixes: 3993f2cb983b ("samples/bpf: Add support for SKB_MODE to xdp1 and xdp_tx_iptunnel")
Signed-off-by: Jesper Dangaard Brouer <[email protected]>
Acked-by: Daniel Borkmann <[email protected]>
Reviewed-by: Andy Gospodarek <[email protected]>
Signed-off-by: David S. Miller <[email protected]>