CVE-2015-666: Revert "sched/x86_64: Don't save flags on context switch"

CVE-2015-666: Revert "sched/x86_64: Don't save flags on context switch"

This reverts commit 2c7577a7583747c9b71f26dced7f696b739da745.

CVE Request: Linux x86_64 NT flag issue

When I fixed Linux's NT flag handling, I added an optimization to
Linux 3.19 and up.  A malicious 32-bit program might be able to leak
NT into an unrelated task.  On a CONFIG_PREEMPT=y kernel, this is a
straightforward DoS.  On a CONFIG_PREEMPT=n kernel, it's probably
still exploitable for DoS with some more care.

I believe that this could be used for privilege escalation, too, but
it won't be easy.

The fix is just to revert the optimization:

Orabug: 21689349
CVE:  CVE-2015-666

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>