libbpf: Set NLM_F_EXCL when creating qdisc

This got lost during the refactoring across versions. We always use
NLM_F_EXCL when creating some TC object, so reflect what the function
says and set the flag.

Fixes: 715c5ce4 ("libbpf: Add low level TC-BPF management API")
Signed-off-by: NKumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210612023502.1283837-3-memxor@gmail.com

tools/lib/bpf/netlink.c

@@ -457,7 +457,7 @@ static int tc_qdisc_modify(struct bpf_tc_hook *hook, int cmd, int flags)
 
 static int tc_qdisc_create_excl(struct bpf_tc_hook *hook)
 {
-	return tc_qdisc_modify(hook, RTM_NEWQDISC, NLM_F_CREATE);
+	return tc_qdisc_modify(hook, RTM_NEWQDISC, NLM_F_CREATE | NLM_F_EXCL);
 }
 
 static int tc_qdisc_delete(struct bpf_tc_hook *hook)