LU-467 Not reinitialize "cl_io" in "llu_file_prwv()"

LU-467 Not reinitialize "cl_io" in "llu_file_prwv()"

In "llu_file_prwv()", the calling "ccc_env_thread_io(env)" will
erases all valid values assigned before into "llu_file_prwv()",
then the "io->ci_obj" and "io->ci_lockreq" become unspecified,
and then causes subsequent "cl_object_top()" to access NULL pointer
and triggers "slp_io_rw_lock()" assertion.

Change-Id: I0e878c39d6fb21c5f4d93d037763ae68ee8f102e
Signed-off-by: nasf <yong.fan@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1031
Reviewed-by: Jinshan Xiong <jay@whamcloud.com>
Reviewed-by: wangdi <di.wang@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>