fs: add put_mnt_ns() cleanup helper

Add a simple helper to put a mount namespace reference.

Link: https://lore.kernel.org/r/20240719-work-mount-namespace-v1-3-834113cab0d2@kernel.org
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/linux/mnt_namespace.h

@@ -3,6 +3,9 @@
 #define _NAMESPACE_H_
 #ifdef __KERNEL__
 
+#include <linux/cleanup.h>
+#include <linux/err.h>
+
 struct mnt_namespace;
 struct fs_struct;
 struct user_namespace;
@@ -11,6 +14,7 @@ struct ns_common;
 extern struct mnt_namespace *copy_mnt_ns(unsigned long, struct mnt_namespace *,
 		struct user_namespace *, struct fs_struct *);
 extern void put_mnt_ns(struct mnt_namespace *ns);
+DEFINE_FREE(put_mnt_ns, struct mnt_namespace *, if (!IS_ERR_OR_NULL(_T)) put_mnt_ns(_T))
 extern struct ns_common *from_mnt_ns(struct mnt_namespace *);
 
 extern const struct file_operations proc_mounts_operations;