LU-5263 llite: update ras stride offset

LU-5263 llite: update ras stride offset

When a read ahead does not reach the end of the region reserved from
ras, we'd set ras::ras_next_readahead back to where we left off; For
stride read ahead, it needs to make sure that the offset is no less
than ras_stride_offset, so that the stride read ahead can work
correctly.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I4996f0d8174fa72ea4ef5994daddc0ab2f922f38
Reviewed-on: http://review.whamcloud.com/11181
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>