x86/reboot: VMCLEAR active VMCSes before emergency reboot

mainline inclusion
from mainline-v6.6-rc1
commit b23c83ad
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I95B2J
CVE: CVE-2023-52514

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit?id=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: Wei Li <liwei391@huawei.com>