sched_getattr: port to copy_struct_to_user

sched_getattr(2) doesn't care about trailing non-zero bytes in the
(ksize > usize) case, so just use copy_struct_to_user() without checking
ignored_trailing.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/r/20241010-extensible-structs-check_fields-v3-2-d2833dfe6edd@cyphar.com
Signed-off-by: Christian Brauner <brauner@kernel.org>