btrfs: Remove fs_info from btrfs_inc_block_group_ro

It can be referenced from the passed bg cache.

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