Merge: netfilter: conntrack: helper: Replace -EEXIST by -EBUSY

MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-10/-/merge_requests/1421

```
JIRA: https://issues.redhat.com/browse/RHEL-108858
Upstream Status: commit 54416fd

commit 54416fd
Author: Phil Sutter <phil@nwl.cc>
Date:   Mon Aug 18 13:22:20 2025 +0200

    netfilter: conntrack: helper: Replace -EEXIST by -EBUSY

    The helper registration return value is passed-through by module_init
    callbacks which modprobe confuses with the harmless -EEXIST returned
    when trying to load an already loaded module.

    Make sure modprobe fails so users notice their helper has not been
    registered and won't work.

    Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
    Fixes: 12f7a50 ("netfilter: add user-space connection tracking helper infrastructure")
    Signed-off-by: Phil Sutter <phil@nwl.cc>
    Signed-off-by: Florian Westphal <fw@strlen.de>

```
Signed-off-by: Phil Sutter <psutter@redhat.com>

Approved-by: Florian Westphal <fwestpha@redhat.com>
Approved-by: Guillaume Nault <gnault@redhat.com>
Approved-by: CKI KWF Bot <cki-ci-bot+kwf-gitlab-com@redhat.com>

Merged-by: CKI GitLab Kmaint Pipeline Bot <26919896-cki-kmaint-pipeline-bot@users.noreply.gitlab.com>

Author: CKI KWF Bot

net/netfilter/nf_conntrack_helper.c

@@ -368,7 +368,7 @@ int nf_conntrack_helper_register(struct nf_conntrack_helper *me)
 			    (cur->tuple.src.l3num == NFPROTO_UNSPEC ||
 			     cur->tuple.src.l3num == me->tuple.src.l3num) &&
 			    cur->tuple.dst.protonum == me->tuple.dst.protonum) {
-				ret = -EEXIST;
+				ret = -EBUSY;
 				goto out;
 			}
 		}
@@ -379,7 +379,7 @@ int nf_conntrack_helper_register(struct nf_conntrack_helper *me)
 		hlist_for_each_entry(cur, &nf_ct_helper_hash[h], hnode) {
 			if (nf_ct_tuple_src_mask_cmp(&cur->tuple, &me->tuple,
 						     &mask)) {
-				ret = -EEXIST;
+				ret = -EBUSY;
 				goto out;
 			}
 		}