fs: ensure that *path_locked*() helpers leave passed path pristine

The functions currently leaving dangling pointers in the passed-in path
leading to hard to debug bugs in the long run. Ensure that the path is
left in pristine state just like we do in e.g., path_parentat() and
other helpers.

Link: https://lore.kernel.org/20250414-rennt-wimmeln-f186c3a780f1@brauner


Signed-off-by: Christian Brauner <brauner@kernel.org>