LU-8465 e2fsck: create logs for mult-threads

LU-8465 e2fsck: create logs for mult-threads

When multi-threads are used, different logs should be created
for different threads. Each thread has log files with suffix
of ".$THREAD_INDEX".

And this patch adds f_multithread_logfile test case.

Change-Id: I9d108f385a1edf06eb0fa67a311856ca9ab0d004
Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/35780
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>