btrfs-progs: convert: Open the fs readonly for rollback

btrfs-progs: convert: Open the fs readonly for rollback

For rollback, we only needs to open the fs to check if it meets the
condition to rollback.  And this RW read makes us failed to rollback
btrfs with v2 space cache.

In fact, we don't even start a transaction during rollback.

So open the fs RO for rollback, to avoid v2 space cache problem.

Reported-by: Gu Jinxiang <gujx@cn.fujitsu.com>
Reviewed-by: Gu JinXiang <gujx@cn.fujitsu.com>
Tested-by: Gu JinXiang <gujx@cn.fujitsu.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>