x86, syscall: Fix _TIF_NOHZ handling in syscall_trace_enter_phase1

TIF_NOHZ is 19 (i.e. _TIF_SYSCALL_TRACE | _TIF_NOTIFY_RESUME |
_TIF_SINGLESTEP), not (1<<19).

This code is involved in Dave's trinity lockup, but I don't see why
it would cause any of the problems he's seeing, except inadvertently
by causing a different path through entry_64.S's syscall handling.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Jones <davej@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/a6cd3b60a3f53afb6e1c8081b0ec30ff19003dd7.1416434075.git.luto@amacapital.net
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/ptrace.c

@@ -1484,7 +1484,7 @@ unsigned long syscall_trace_enter_phase1(struct pt_regs *regs, u32 arch)
 	 */
 	if (work & _TIF_NOHZ) {
 		user_exit();
-		work &= ~TIF_NOHZ;
+		work &= ~_TIF_NOHZ;
 	}
 
 #ifdef CONFIG_SECCOMP