x86/spectre_v2: Do not disable IBPB when disabling IBRS

x86/spectre_v2: Do not disable IBPB when disabling IBRS

Upstream has decided that while IBRS is bad, IBPB is good.

In fact:
18bf3c3ea8ece8f03b6fc58508f2dfd23c7711c7 x86/speculation: Use Indirect Branch Prediction Barrier in context switch

and KVM patches:
15d45071523d89b3fb7372e2135fbd72f6af9506 KVM/x86: Add IBPB support

all use indirect_branch_prediction_barrier().

In our code base the indirect_branch_prediction_barrier
is wrapped with an check:

if (ibpb_inuse)
	wrmsrl(MSR_IA32_PRED_CMD, FEATURE_SET_IBPB);

But nonethless we should keep the IBPB disabled on the normal path.

However if folks have choosen 'spectre_v2=off' or 'spectre_v2=none'
then we MUST disable the IBPB.

Orabug: 27477743
CVE: CVE-2017-5715

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>