log/file_output: fix and clean up the file output callout
log/file_output: fix and clean up the file output callout

Signed-off-by: Xiubo Li <67e5e1ee902b4e67bce18ea43d9c4b0430ce5e6c@cmss.chinamobile.com>

open-iscsi/tcmu-runner,open-iscsi/tcmu-runner,agrover/tcmu-runner