We also check user pointer in x86_64 put_user, the way i386 does.

In a separate patch for bisecting purposes.
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>