LU-12043 llite: make sure readahead cover current read

LU-12043 llite: make sure readahead cover current read

When doing readahead, @ria_end_min is used to indicate
how far we are expected to read to cover current
read.

update @ria_end_min unconditionally with IO end.
also @ria_end_min is closed interval which should be
calculated as start + count - 1;

Change-Id: If7f8da44da31623a73b363d5a18c1ec8b54da745
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/35215
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>