LU-1059 clio: to not try to discard freeing pages

LU-1059 clio: to not try to discard freeing pages

This is a bug imported in LU-948. We should check if we have owned
the page successfully before trying to discard it.

Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Change-Id: I30631be98f1fcc1b98abe727c8c6984b918bfffd
Reviewed-on: http://review.whamcloud.com/2073
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>