LU-9574 llite: pipeline readahead better with large I/O

LU-9574 llite: pipeline readahead better with large I/O

Fixed a bug where next readahead is not set correctly when
appplication issues large I/O;
Extend the readahead window length to at least cover the size of
current I/O.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I43c5e4f25ea30d4a36263db2588bde0401122990
Reviewed-on: https://review.whamcloud.com/27388
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>