printk/console: Check consistent sequence number when handling race in console_unlock()

The standard printk() tries to flush the message to the console
immediately. It tries to take the console lock. If the lock is
already taken then the current owner is responsible for flushing
even the new message.

There is a small race window between checking whether a new message is
available and releasing the console lock. It is solved by re-checking
the state after releasing the console lock. If the check is positive
then console_unlock() tries to take the lock again and process the new
message as well.

The commit 996e9666 ("printk: remove logbuf_lock") causes that
console_seq is not longer read atomically. As a result, the re-check might
be done with an inconsistent 64-bit index.

Solve it by using the last sequence number that has been checked under
the console lock. In the worst case, it will take the lock again only
to realized that the new message has already been proceed. But it
was possible even before.

The variable next_seq is marked as __maybe_unused to call down compiler
warning when CONFIG_PRINTK is not defined.

Fixes: commit 996e9666

 ("printk: remove logbuf_lock")
Reported-by: kernel test robot <lkp@intel.com>  # unused next_seq warning
Cc: stable@vger.kernel.org # 5.13
Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Link: https://lore.kernel.org/r/20210702150657.26760-1-pmladek@suse.com