libbpf: Use OPTS_SET() macro in bpf_xdp_query()

When the feature_flags and xdp_zc_max_segs fields were added to the libbpf
bpf_xdp_query_opts, the code writing them did not use the OPTS_SET() macro.
This causes libbpf to write to those fields unconditionally, which means
that programs compiled against an older version of libbpf (with a smaller
size of the bpf_xdp_query_opts struct) will have its stack corrupted by
libbpf writing out of bounds.

The patch adding the feature_flags field has an early bail out if the
feature_flags field is not part of the opts struct (via the OPTS_HAS)
macro, but the patch adding xdp_zc_max_segs does not. For consistency, this
fix just changes the assignments to both fields to use the OPTS_SET()
macro.

Fixes: 13ce2daa259a ("xsk: add new netlink attribute dedicated for ZC max frags")
Signed-off-by: Toke Høiland-Jørgensen <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/netlink.c

···
 	if (err)
 		return libbpf_err(err);
 
-	opts->feature_flags = md.flags;
-	opts->xdp_zc_max_segs = md.xdp_zc_max_segs;
+	OPTS_SET(opts, feature_flags, md.flags);
+	OPTS_SET(opts, xdp_zc_max_segs, md.xdp_zc_max_segs);
 
 skip_feature_flags:
 	return 0;