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

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.
test 24 of recovery-small is now reenabled back.

port of master patch 624a3ac233938153e889ae81f639cf46c0a8312b
port of master patch 4cfcf91efa06decc155d8f12eafc8ede3e850fd5

Signed-off-by: Hongchao Zhang <hongchao.zhang@whamcloud.com>
Signed-off-by: Liu Ying <emoly.liu@intel.com>
Change-Id: If8aeed507890a656cbb8231879d0e31aa9f4ea02
Reviewed-on: http://review.whamcloud.com/4716
Tested-by: Hudson
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>