LU-8465 e2fsck: add start/end group for thread

LU-8465 e2fsck: add start/end group for thread

When multi-threads are used for check, each thread needs to jump
to different group in pass1 check. This patch adds the group
jumping support. But still, only one thread is used to check.

Change-Id: I3c299f4383f0406782fbedf77cd2641a8131db49
Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/35846
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>