printk: re-organize log_output() to be more legible

Avoid some duplicate logic now that we can return early, and update the
comments for the new LOG_CONT world order.

This also stops the continuation flushing from just using random record
flags for the flushing action, instead taking the flags from the proper
original line and updating them as we add continuations to it.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>