x86/KVM/VMX: Add module argument for L1TF mitigation

x86/KVM/VMX: Add module argument for L1TF mitigation

Add a mitigation mode parameter "vmentry_l1d_flush" for CVE-2018-3646, aka
L1 terminal fault. The valid arguments are:

 - "always" 	L1D cache flush on every VMENTER.
 - "cond"	Conditional L1D cache flush, explained below
 - "never"	Disable the L1D cache flush mitigation

"cond" is trying to avoid L1D cache flushes on VMENTER if the code executed
between VMEXIT and VMENTER is considered safe, i.e. is not bringing any
interesting information into L1D which might exploited.

[ tglx: Split out from a larger patch ]

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Orabug: 28220674
CVE: CVE-2018-3646

(cherry picked from commit a399477e52c17e148746d3ce9a483f681c2aa9a0)

Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Conflicts:
	arch/x86/kvm/vmx.c
	Documentation/admin-guide/kernel-parameters.txt
Contextual: different content.
Replaced DEFINE_STATIC_KEY_FALSE and static_branch_enable with their
existent equivalent and cherry-picked e33886b38.