printk: nbcon: Show replay message on takeover

printk: nbcon: Show replay message on takeover

An emergency or panic context can takeover console ownership
while the current owner was printing a printk message. The
atomic printer will re-print the message that the previous
owner was printing. However, this can look confusing to the
user and may even seem as though a message was lost.

  [3430014.1
  [3430014.181123] usb 1-2: Product: USB Audio

Add a new field @nbcon_prev_seq to struct console to track
the sequence number to print that was assigned to the previous
console owner. If this matches the sequence number to print
that the current owner is assigned, then a takeover must have
occurred. In this case, print an additional message to inform
the user that the previous message is being printed again.

  [3430014.1
  ** replaying previous printk message **
  [3430014.181123] usb 1-2: Product: USB Audio

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240904120536.115780-12-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>