Btrfs-progs: fsck: deal with really corrupted extent tree

Btrfs-progs: fsck: deal with really corrupted extent tree

To reinit extent root, we need find a free extent, however,
we may have a really corrupted extent tree, so we can't rely
on existed extent tree to cache block group any more.

During test, we fail to reinit extent tree which is because we
can not find a free extent so let's make block group cache ourselves
firstly.

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>