samples/bpf: run cleanup routines when receiving SIGTERM

Shahid Habib noticed that when xdp1 was killed from a different console the xdp
program was not cleaned-up properly in the kernel and it continued to forward
traffic.

Most of the applications in samples/bpf cleanup properly, but only when getting
SIGINT.  Since kill defaults to using SIGTERM, add support to cleanup when the
application receives either SIGINT or SIGTERM.

Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Reported-by: Shahid Habib <shahid.habib@broadcom.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

samples/bpf/tracex2_user.c

@@ -127,6 +127,7 @@ int main(int ac, char **argv)
 	}
 
 	signal(SIGINT, int_exit);
+	signal(SIGTERM, int_exit);
 
 	/* start 'ping' in the background to have some kfree_skb events */
 	f = popen("ping -c5 localhost", "r");