LU-8465 e2fsck: optionally configure one pfsck thread

LU-8465 e2fsck: optionally configure one pfsck thread

This patch creates only one thread to do pass1 check. The same
codes can be used to create multiple threads, but other functions
need to be modified to get ready for that.

pfsck support will be enabled with if configured with
--enable-pfsck option.

Change-Id: Ic1c704d357aa323b19211fbefb9ab6dbd798b9aa
Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/35793
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>