x86/reboot: VMCLEAR active VMCSes before emergency reboot

x86/reboot: VMCLEAR active VMCSes before emergency reboot

[ Upstream commit b23c83ad2c638420ec0608a9de354507c41bec29 ]

VMCLEAR active VMCSes before any emergency reboot, not just if the kernel
may kexec into a new kernel after a crash.  Per Intel's SDM, the VMX
architecture doesn't require the CPU to flush the VMCS cache on INIT.  If
an emergency reboot doesn't RESET CPUs, cached VMCSes could theoretically
be kept and only be written back to memory after the new kernel is booted,
i.e. could effectively corrupt memory after reboot.

Opportunistically remove the setting of the global pointer to NULL to make
checkpatch happy.

Cc: Andrew Cooper <Andrew.Cooper3@citrix.com>
Link: https://lore.kernel.org/r/20230721201859.2307736-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>