path: add cleanup helper

BugLink: https://bugs.launchpad.net/bugs/2085849

[ Upstream commit ff2c570ef7eaa9ded58e7a02dd7a68874a897508 ]

Add a simple cleanup helper so we can cleanup struct path easily.
No need for any extra machinery. Avoid DEFINE_FREE() as it causes a
local copy of struct path to be used. Just rely on path_put() directly
called from a cleanup helper.

Link: https://lore.kernel.org/r/20240607-vfs-listmount-reverse-v1-2-7877a2bfa5e5@kernel.org
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Stable-dep-of: dd7cb142f467 ("fs: relax permissions for listmount()")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Manuel Diewald <manuel.diewald@canonical.com>
Signed-off-by: Roxana Nicolescu <roxana.nicolescu@canonical.com>

include/linux/path.h

@@ -24,4 +24,13 @@ static inline void path_put_init(struct path *path)
 	*path = (struct path) { };
 }
 
+/*
+ * Cleanup macro for use with __free(path_put). Avoids dereference and
+ * copying @path unlike DEFINE_FREE(). path_put() will handle the empty
+ * path correctly just ensure @path is initialized:
+ *
+ * struct path path __free(path_put) = {};
+ */
+#define __free_path_put path_put
+
 #endif  /* _LINUX_PATH_H */