readahead02: fix signed/unsigned comparison warnings

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>
readahead02: read in 2M chunks

Max readahead size has been limited since commit:
  commit 6d2be915e589b58cb11418cbe1f22ff90732b6ac
  Author: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
  Date:   Thu Apr 3 14:48:23 2014 -0700
    mm/readahead.c: fix readahead failure for memoryless NUMA nodes
                    and limit readahead pages

Update testcase to read ahead in 2M chunks, this change is
compatible with older kernels.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>