printk: queue wake_up_klogd irq_work only if per-CPU areas are ready

Hello,

Commit ab6f762f0f53162d41 Linus' HEAD.

printk_deferred() does not make sure that it's safe to write to per-CPU data, which causes problems when printk_deferred() is invoked "too early", before per-CPU areas are initialized. There are multiple bug reports, e.g. https://bugzilla.kernel.org/show_bug.cgi?id=206847

-ss