Btrfs-progs: don't fail on log tree opening with PARTIAL

We were failing to fsck a volume because we couldn't open the log tree, which is
not helpful.  Make us skip erroring out if we are using OPEN_CTREE_PARTIAL since
it isn't a mandatory tree.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.cz>

disk-io.c

@@ -937,6 +937,7 @@ int btrfs_setup_all_roots(struct btrfs_fs_info *fs_info, u64 root_tree_bytenr,
 	ret = find_and_setup_log_root(root, fs_info, sb);
 	if (ret) {
 		printk("Couldn't setup log root tree\n");
+		if (!(flags & OPEN_CTREE_PARTIAL))
 			return -EIO;
 	}