s390/con3270: add special output handling when oops_in_progress is set

Normally a user can scroll back with PF7/PF8 if printed messages are
outside of the visible screen area. This doesn't work when the kernel
crashes, because the scrollback handling is done by the kernel, which
is no longer alive after the kernel crash. Add code to always print
all dirty lines in the screen buffer, so the user can scroll back with
the terminal scrollback keys (Page Up/Down).

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>