tty: vt: replace BUG_ON() by WARN_ON_ONCE()

tty: vt: replace BUG_ON() by WARN_ON_ONCE()

No need to panic in vc_uniscr_copy_line(), just warn. This should never
happen though, as vc_uniscr_check() is supposed to be called before
vc_uniscr_copy_line(). And the former checks vc->vc_uni_lines already.

In any case, use _ONCE as vc_uniscr_copy_line() is called repeatedly for
each line. So don't flood the logs, just in case.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20230112080136.4929-7-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>