procfs: fix a locking bug in a vmcore_add_device_dump() error path

Unlock vmcore_mutex when returning -EBUSY.

Link: https://lkml.kernel.org/r/20250129222003.1495713-1-bvanassche@acm.org
Fixes: 0f3b1c4 ("fs/proc/vmcore: disallow vmcore modifications while the vmcore is open")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Baoquan he <bhe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: bvanassche

fs/proc/vmcore.c

@@ -1524,14 +1524,17 @@ int vmcore_add_device_dump(struct vmcoredd_data *data)
 		pr_warn_once("Unexpected adding of device dump\n");
 	if (vmcore_open) {
 		ret = -EBUSY;
-		goto out_err;
+		goto unlock;
 	}
 
 	list_add_tail(&dump->list, &vmcoredd_list);
 	vmcoredd_update_size(data_size);
 	mutex_unlock(&vmcore_mutex);
 	return 0;
 
+unlock:
+	mutex_unlock(&vmcore_mutex);
+
 out_err:
 	vfree(buf);
 	vfree(dump);