x86: move __addr_ok to uaccess.h.

Take it out of uaccess_32.h. Since it seems that no users
of the x86_64 exists, we simply pick the i386 version.

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>