LU-1421 clio: check cp_owner of cl_page after the page is owned

LU-1421 clio: check cp_owner of cl_page after the page is owned

After page writeback is supported in clio, there exists a short
period that page is unlocked but cp_owner is set. This confuses
the assertion of cl_page_assume(). This problem can be fixed by
checking cp_owner after the page is owned.

Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Change-Id: Iafcdbb9b0dbb5f081a84c1da1b7ad8051d406988
Reviewed-on: http://review.whamcloud.com/3027
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>