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>

include/uapi/linux/fs.h

@@ -151,7 +151,7 @@ struct fsxattr {
 /*
  * Variable size structure for file_[sg]et_attr().
  *
- * Note. This is alternative to the structure 'struct fileattr'/'struct fsxattr'.
+ * Note. This is alternative to the structure 'struct file_kattr'/'struct fsxattr'.
  * As this structure is passed to/from userspace with its size, this can
  * be versioned based on the size.
  */