fhandle: rename to get_path_anchor()

Rename as we're going to expand the function in the next step. The path
just serves as the anchor tying the decoding to the filesystem.

Link: https://lore.kernel.org/20250624-work-pidfs-fhandle-v2-3-d02a04858fe3@kernel.org


Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>