x86: Restrict MSR access when the kernel is locked down

Writing to MSRs should not be allowed if the kernel is locked down, since
it could lead to execution of arbitrary code in kernel mode.  Based on a
patch by Kees Cook.

Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>
Signed-off-by: David Howells <dhowells@redhat.com>