pidfs: switch to copy_struct_to_user()

We have a helper that deals with all the required logic.

Link: https://lore.kernel.org/r/20250305-work-pidfs-kill_on_last_close-v3-1-c8c3d8361705@kernel.org


Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>