tty/vt: consolemap: extract con_allocate_new() from con_do_clear_unimap()

The first part of con_do_clear_unimap() is needed on another place, so
extract it to a separate function called con_allocate_new(). It will be
used once more in the next patch.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220607104946.18710-34-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>