LU-1109 llite: do splice read stripe by stripe

LU-1109 llite: do splice read stripe by stripe

If nfsd is reading an across stripe buffer, and if the first stripe
happens to be 64KB(PIPE_BUFFERS*PAGE_SIZE), then first read will
occupy all pipe buffers and this makes nfsd stuck if it reads the
next stripe immediately.

Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Change-Id: I13cb54b37f738ee3c081dff1929630ea523b77fd
Reviewed-on: http://review.whamcloud.com/2182
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>