BugLink: https://bugs.launchpad.net/bugs/1830176

Add info about the default mitigation which is part of upstream commit
f21b53b2 ("x86/speculation: Make "seccomp" the default mode for
Speculative Store Bypass").

Fixes: 37e59563 ("UBUNTU: SAUCE: x86/speculation: Make "seccomp" the default mode for Speculative Store Bypass")
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>