LU-12518 readahead: convert stride page index to byte

LU-12518 readahead: convert stride page index to byte

This is a prepared patch to support unaligned stride readahead.
Some detection variables are converted to byte unit to be aware
of possible unaligned stride read.

Since we still need read pages by page index, so those variables
are still kept as page unit. to make things more clear, fix them
to use pgoff_t rather than unsigned long.

Change-Id: Ic0d0b1b2f11cb6ef5753814a5b8fc6d34afb25ea
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/35829
Reviewed-by: Li Xi <lixi@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>