x86: spec_ctrl: fix SPEC_CTRL initialization after kexec

We can't assume the SPEC_CTRL msr is zero at boot because it could be
left enabled by a previous kernel booted with
spec_store_bypass_disable=on.

Without this fix a boot with spec_store_bypass_disable=on followed by
a kexec boot with spec_store_bypass_disable=off would erroneously and
unexpectedly leave bit 2 set in SPEC_CTRL.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>