LU-12518 llite: fix stride window increase

LU-12518 llite: fix stride window increase

Fix following problems:

1. stride_byte_count() argument @off should be @windows_start
rather than @stride_offset to calculate stride bytes.

2. In a limited memory client(for testing etc), we could possibly
have ra_rpc_size(64M) initially > ra_max_pages_per_file, this will
make possibly @window_len 0 after ras_increase_window()

3. @window_len in ras_stride_increase_window() could be negative,
be carefully to avoid overflow.

Change-Id: Ied00bec834d4bb0ad04b688c10a03bbcd667f39b
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/35893
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>