LU-12644 llite: try fast io for stride io correctly

We could have a really large gap for stride, calculate
skip pages correctly, otherwise, we will see many small
RPC with large stride gap.

Change-Id: Id72405c11234a2075f3cce4733d23544fe15eb17
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/35466
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>