btrfs: reloc: add backref_cache::fs_info member

Add this member so that we can grab fs_info without the help from
reloc_control.

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