btrfs: add a wbc pointer to struct btrfs_bio_ctrl

Instead of passing down the wbc pointer the deep call chain, just
add it to the btrfs_bio_ctrl structure.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>