x86/mm: Only set IBPB when the new thread cannot ptrace current thread

x86/mm: Only set IBPB when the new thread cannot ptrace current thread

CVE-2017-5753
CVE-2017-5715

To reduce overhead of setting IBPB, we only do that when
the new thread cannot ptrace the current one.  If the new
thread has ptrace capability on current thread, it is safe.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>