tree-wide: s/struct fileattr/struct file_kattr/g

Now that we expose struct file_attr as our uapi struct rename all the
internal struct to struct file_kattr to clearly communicate that it is a
kernel internal struct. This is similar to struct mount_{k}attr and
others.

Link: https://lore.kernel.org/20250703-restlaufzeit-baurecht-9ed44552b481@brauner


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