x86/ptrace: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

Fix the following warning (Building: allnoconfig i386):

arch/x86/kernel/ptrace.c:202:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (unlikely(value == 0))
      ^
arch/x86/kernel/ptrace.c:206:2: note: here
  default:
  ^~~~~~~

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20190805195654.GA17831@embeddedor

arch/x86/kernel/ptrace.c

@@ -201,6 +201,7 @@ static int set_segment_reg(struct task_struct *task,
 	case offsetof(struct user_regs_struct, ss):
 		if (unlikely(value == 0))
 			return -EIO;
+		/* Else, fall through */
 
 	default:
 		*pt_regs_access(task_pt_regs(task), offset) = value;