e2fsck: copy fs when using multi-thread fsck

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.

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>