nsproxy: add a cleanup helper for nsproxy

Add a simple cleanup helper for nsproxy.

Link: https://lore.kernel.org/r/20240627-work-pidfs-v1-2-7e9ab6cc3bb1@kernel.orgReviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/linux/nsproxy.h

@@ -112,4 +112,6 @@ static inline void get_nsproxy(struct nsproxy *ns)
 	refcount_inc(&ns->count);
 }
 
+DEFINE_FREE(put_nsproxy, struct nsproxy *, if (_T) put_nsproxy(_T))
+
 #endif