LU-12043 llite, readahead: fix to call ll_ras_enter() properly

LU-12043 llite, readahead: fix to call ll_ras_enter() properly

ll_ras_enter() is expected to be called per syscall.
However, with fast read enabled, it will be no longer true that
We will call vvp_io_read_start() for every syscall.

To fix this problem, we should move this to file read handler.

Change-Id: I8d70714b2e8bc04b7c4ab996d189f10f37488d97
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/34755
Tested-by: Jenkins
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@gmail.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>