selftests/bpf: Fix xdp_tx.c prog section name

The program type cannot be deduced from 'tx' which causes an invalid
argument error when trying to load xdp_tx.o using the skeleton.
Rename the section name to "xdp" so that libbpf can deduce the type.
Signed-off-by: Jussi Maki <joamaki@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210731055738.16820-7-joamaki@gmail.com

tools/testing/selftests/bpf/progs/xdp_tx.c

@@ -3,7 +3,7 @@
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
 
-SEC("tx")
+SEC("xdp")
 int xdp_tx(struct xdp_md *xdp)
 {
 	return XDP_TX;

tools/testing/selftests/bpf/test_xdp_veth.sh

@@ -108,7 +108,7 @@ ip link set dev veth2 xdp pinned $BPF_DIR/progs/redirect_map_1
 ip link set dev veth3 xdp pinned $BPF_DIR/progs/redirect_map_2
 
 ip -n ns1 link set dev veth11 xdp obj xdp_dummy.o sec xdp_dummy
-ip -n ns2 link set dev veth22 xdp obj xdp_tx.o sec tx
+ip -n ns2 link set dev veth22 xdp obj xdp_tx.o sec xdp
 ip -n ns3 link set dev veth33 xdp obj xdp_dummy.o sec xdp_dummy
 
 trap cleanup EXIT