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.

This patch is back-ported from the following one:
Lustre-commit: 9e280281b8daf8ad69d04a5371121224ace28fbc
Lustre-change: http://review.whamcloud.com/11181

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I5d319a94435833153c52da6ccc1b20c4952789e9
Signed-off-by: Jian Yu <jian.yu@intel.com>
Reviewed-on: http://review.whamcloud.com/11455
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>