e2fsck: create logs for mult-threads

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.

E2fsprogs-commit: 2643de7d00dc8da9f5be79fab768fca7a27f3745

Change-Id: I86982866b9e8e2d2cc5ac2ec9ac44a3b802c21e0
Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>