LU-13412 llite: fix read if readahead window smaller than rpc size

LU-13412 llite: fix read if readahead window smaller than rpc size

Readahead always try to align readahead with RPC size, but this
could introduce a problem if readahead window is smaller than RPC size.

With current codes, it will fallback a lot of 4k read because
RPC aligned window start plus window pages will be behind of
current read. Fix this to align with readahead window rather
than RPC size in this case.

Change-Id: I0cd33ac7f92a75f38c926db33630f3036bbfd6c7
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/38132
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>