x86: ia32_signal: use __put_user() instead of __copy_to_user()

Impact: cleanup

__put_user() can be used for constant size 8, like arch/x86/kernel/signal.c.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>