CVE-2017-5753
CVE-2017-5715

Stuff RSB to prevent RSB underflow on non-SMEP platform.
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
(backported from commit 183ab2f8dfb26ad2c83602af3ee9a5f11d65128b)
Signed-off-by: Andy Whitcroft <apw@canonical.com>