fuse: __kuid_val/__kgid_val helpers in fuse_fill_attr_from_inode()

For the sake of consistency, let's use these helpers to extract
{u,g}id_t values from k{u,g}id_t ones.

There are no functional changes, just to make code cleaner.

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>