zram_drv: allow overriding zram size from kernel

Add a CONFIG_ZRAM_SIZE_OVERRIDE option, size will be set
to 1 GB multiply by CONFIG_ZRAM_SIZE_OVERRIDE.

Signed-off-by: Dmitry Eliseev <dmitry.e@cloudberrylab.com>
Signed-off-by: Yaroslav Furman <yaro330@gmail.com>