Move the RSB_CTXSW hunk further up in spectre_v2_select_mitigation() to
match upstream. No functional changes.

CVE-2017-5715
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>