btrfs: don't pass io_ctl to __btrfs_write_out_cache()

btrfs: don't pass io_ctl to __btrfs_write_out_cache()

There's no need to pass both the block_group and block_group::io_ctl to
__btrfs_write_out_cache().

Remove passing io_ctl to __btrfs_write_out_cache() and dereference it
inside __btrfs_write_out_cache().

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>