LU-8465 e2fsck: copy fs when using multi-thread fsck

LU-8465 e2fsck: copy fs when using multi-thread fsck

This patch only copy the fs to a new one when -m is enabled.
It doesn't actually start any thread. When pass1 test finishes,
the new fs is copied back to the original context.

This patch handles the fs fields in dblist, inode_map and block_map
properly.

Change-Id: I1cc80bb1d762692a6bde3a57d1498d2842884c41
Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/35696
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>