Btrfs-progs: fsck: handle case that we can not lookup extent info

Btrfs-progs: fsck: handle case that we can not lookup extent info

Previously, --init-extent-tree works just because btrfs_lookup_extent_info()
blindly return 0, and this make it work if there are not any *FULL BACKREF*
mode in broken filesystem.

It is just a coincidence that --init-extent-tree option works, let's
do it in the right way firstly.

For now, we have not supported to rebuild extent tree if there are
any *FULL BACKREF* mode which means if there are snapshots with broken
filesystem, avoid using --init-extent-tree option now.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>