x86: ds.c ptrace.c integer as NULL pointer sparse fixes

fix:

 arch/x86/kernel/ptrace.c:763:29: warning: Using plain integer as NULL pointer
 arch/x86/kernel/ptrace.c:777:46: warning: Using plain integer as NULL pointer
 arch/x86/kernel/ptrace.c:1115:45: warning: Using plain integer as NULL pointer
 arch/x86/kernel/ds.c:482:26: warning: Using plain integer as NULL pointer
 arch/x86/kernel/ds.c:487:25: warning: Using plain integer as NULL pointer

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>