tty: vt, unify scrolling functions

Both scrup and scrdown are copies of the same code except source and
destination pointers computation. Unify those functions into a single
one named con_scroll.

Note that scrdown used step to compute the destination, while scrup
did the computation explicitly. We sticked to the latter here.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>