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-2017-5715 (Spectre v2 Intel)

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>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>