LU-921 llite: warning in case of discarding dirty pages

when a client is evicted, dirty pages may get silently discarded,
the caller of successful write(2) will not know that the data he
wrote have been discarded due to eviction before it can be flushed
to the OSS.

Signed-off-by: Hongchao Zhang <hongchao.zhang@whamcloud.com>
Change-Id: Iecfbf096548ff08cdd6064d53ad8c688343fcddc
Reviewed-on: http://review.whamcloud.com/1908
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>