kmsg - do not flush partial lines when the console is busy

Fragments of continuation lines are flushed to the console immediately. In
case the console is locked, the fragment must be queued up in the cont
buffer.

If the the console is busy and the continuation line is complete, but no part
of it was written to the console up to this point, we can just store the
entire line as a regular record and free the buffer earlier.

If the console is busy and earlier messages are already queued up, we
should not flush the fragments of continuation lines, but store them after
the queued up messages, to ensure the proper ordering.

This keeps the console output better readable in case printk()s race against
each other, or we receive over-long continuation lines we need to flush.

Signed-off-by: Kay Sievers <kay@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>