btrfs-progs: find-root: Allow btrfs-find-root to search chunk root even chunk root...

btrfs-progs: find-root: Allow btrfs-find-root to search chunk root even chunk root is corrupted

Since now open_ctree_fs_info() can even return a valid fs_info with only
system chunk mapping from super block, use this ability to do chunk root
search for heavily damanged fs.

As an fast alternative for time consuming and buggy chunk-recovery.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>