x86/speculation/mds: Add BUG_MSBDS_ONLY

x86/speculation/mds: Add BUG_MSBDS_ONLY

This bug bit is set on CPUs which are only affected by Microarchitectural
Store Buffer Data Sampling (MSBDS) and not by any other MDS variant.

This is important because the Store Buffers are partitioned between
Hyper-Threads so cross thread forwarding is not possible. But if a thread
enters or exits a sleep state the store buffer is repartitioned which can
expose data from one thread to the other. This transition can be mitigated.

That means that for CPUs which are only affected by MSBDS SMT can be
enabled, if the CPU is not affected by other SMT sensitive vulnerabilities,
e.g. L1TF. The XEON PHI variants fall into that category.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
CVE-2018-12126
CVE-2018-12127
CVE-2018-12130

(cherry picked from commit ab12e2e20232e9b84e2b212346b9fe3956e91642)
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>