x86/fpu: Keep xfd_state in sync with MSR_IA32_XFD

In the Linux kernel, the following vulnerability has been resolved:

x86/fpu: Keep xfd_state in sync with MSR_IA32_XFD

Commit 672365477ae8 ("x86/fpu: Update XFD state where required") and

commit 8bf26758ca96 ("x86/fpu: Add XFD state to fpstate") introduced a

per CPU variable xfd_state to keep the MSR_IA32_XFD value cached, in

order to avoid unnecessary writes to the MSR.

On CPU hotplug MSR_IA32_XFD is reset to the init_fpstate.xfd, which

wipes out any stale state. But the per CPU cached xfd value is not

reset, which brings them out of sync.

As a consequence a subsequent xfd_update_state() might fail to update

the MSR which in turn can result in XRSTOR raising a #NM in kernel

space, which crashes the kernel.

To fix this, introduce xfd_set_state() to write xfd_state together

with MSR_IA32_XFD, and use it in all places that set MSR_IA32_XFD.