btrfs: ref-verify: handle damaged extent root tree

Syzbot hits a problem with enabled ref-verify, ignorebadroots and a
fuzzed/damaged extent tree. There's no fallback option like in other
places that can deal with it so disable the whole ref-verify as it is
just a debugging feature.

Reported-by:  <syzbot+9c3e0cdfbfe351b0bc0e@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/all/0000000000001b6052062139be1c@google.com/


Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>