x86: use k modifier for 4-byte access.

Do it in a separate patch for bisectability.
Goal is to have put_user_size integrated.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>