libbpf: Adjust SEC short cut for expected attach type BPF_XDP_DEVMAP

Adjust the SEC("xdp_devmap/") prog type prefix to contain a
slash "/" for expected attach type BPF_XDP_DEVMAP.  This is consistent
with other prog types like tracing.

Fixes: 2778797037 ("libbpf: Add SEC name for xdp programs attached to device map")
Suggested-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/159309521882.821855.6873145686353617509.stgit@firesoul

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

@@ -27,7 +27,7 @@ int xdp_dummy_prog(struct xdp_md *ctx)
 /* valid program on DEVMAP entry via SEC name;
  * has access to egress and ingress ifindex
  */
-SEC("xdp_devmap")
+SEC("xdp_devmap/map_prog")
 int xdp_dummy_dm(struct xdp_md *ctx)
 {
 	char fmt[] = "devmap redirect: dev %u -> dev %u len %u\n";