LU-5505 clio: revise read ahead algorithm

LU-5505 clio: revise read ahead algorithm

ras_window_len should only be updated in ras_update() by read
pattern and it can't be adjusted in ll_readahead() at all;
ras_consecutive_pages is used to detect read pattern from
mmap. It will be used to increase read ahead window length
gradually.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I78b41646ccd8d9d1c810196a8cbcf58adbcb9319
Reviewed-on: http://review.whamcloud.com/11528
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>