fsck.f2fs: read ahead xattr & direct node blocks

fsck.f2fs: read ahead xattr & direct node blocks

This patch read ahead xattr & direct node blocks, and
keep the order:

1. check data blocks
2. readahead xattr block
3. fsck xattr block
4. readahead {d,id,did}node block
5. fsck {d,id,did}node block

With above order, we can avoid unneeded readahead before
sub-directory iterated traversing or encountering error.

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: Fix a bug breaking the right order.]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>