pping: Change default ingress program from XDP to tc

Using the XDP ingress hook requires a newer kernel (needs Toke's patch
fixing the verification of global function for BPF_PROG_TYPE_EXT
programs) than tc mode, is will likely perform worse than tc if
running in generic mode (due to no driver support for
XDP). Furthermore, even when XDP works and has driver support, its
performance benefit over tc is likely small as the packets are always
passed on to the network stack regardless (not creating a fast-path
that bypasses the network stack). Therefore, use the tc ingress hook
as default instead, and only use XDP if explicitly required by the
user (-I/--ingress hook xdp).

This partly addresses issue #49, as ePPing should no longer by default
get the confusing error message from failing verification if the
kernel lacks Toke's verifier patch.

Signed-off-by: Simon Sundberg <simon.sundberg@kau.se>

pping/pping.c

@@ -1013,7 +1013,7 @@ int main(int argc, char *argv[])
 		.clean_args = { .cleanup_interval = 1 * NS_PER_SECOND,
 				.valid_thread = false },
 		.object_path = "pping_kern.o",
-		.ingress_prog = PROG_INGRESS_XDP,
+		.ingress_prog = PROG_INGRESS_TC,
 		.egress_prog = PROG_EGRESS_TC,
 		.cleanup_ts_prog = "tsmap_cleanup",
 		.cleanup_flow_prog = "flowmap_cleanup",