x86/entry: Fix _TIF_USER_RETURN_NOTIFY check in prepare_exit_to_usermode

Linus noticed that the early return check was missing
_TIF_USER_RETURN_NOTIFY.  If the only work flag was
_TIF_USER_RETURN_NOTIFY, we'd skip user return notifiers.  Fix
it. (This is the only missing bit.)

This fixes double faults on a KVM host.  It's the same issue as
last time, except that this time it's very easy to trigger.
Apparently no one uses -next as a KVM host.

( I'm still not quite sure what it is that KVM does that blows up
  so badly if we miss a user return notifier.  My best guess is that KVM
  lets KERNEL_GS_BASE (i.e. the user's gs base) be negative and fixes
  it up in a user return notifier.  If we actually end up in user mode
  with a negative gs base, we blow up pretty badly. )
Reported-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NAndy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: c5c46f59 ("x86/entry: Add new, comprehensible entry and exit handlers written in C")
Link: http://lkml.kernel.org/r/3f801104d24ee7a6bb1446408d9950777aa63277.1436995419.git.luto@kernel.orgSigned-off-by: NIngo Molnar <mingo@kernel.org>

arch/x86/entry/common.c

@@ -264,7 +264,8 @@ __visible void prepare_exit_to_usermode(struct pt_regs *regs)
 			READ_ONCE(pt_regs_to_thread_info(regs)->flags);
 
 		if (!(cached_flags & (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME |
-				      _TIF_UPROBE | _TIF_NEED_RESCHED)))
+				      _TIF_UPROBE | _TIF_NEED_RESCHED |
+				      _TIF_USER_RETURN_NOTIFY)))
 			break;
 
 		/* We have work to do. */