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: [e-mail]
Link: https://lore.kernel.org/all/[e-mail]/
Reviewed-by: Qu Wenruo <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/ref-verify.c

···
 	if (!btrfs_test_opt(fs_info, REF_VERIFY))
 		return 0;
 
+	extent_root = btrfs_extent_root(fs_info, 0);
+	/* If the extent tree is damaged we cannot ignore it (IGNOREBADROOTS). */
+	if (IS_ERR(extent_root)) {
+		btrfs_warn(fs_info, "ref-verify: extent tree not available, disabling");
+		btrfs_clear_opt(fs_info->mount_opt, REF_VERIFY);
+		return 0;
+	}
+
 	path = btrfs_alloc_path();
 	if (!path)
 		return -ENOMEM;
 
-	extent_root = btrfs_extent_root(fs_info, 0);
 	eb = btrfs_read_lock_root_node(extent_root);
 	level = btrfs_header_level(eb);
 	path->nodes[level] = eb;