btrfs: store the bio opf in struct btrfs_bio_ctrl

The bio op and flags never change over the life time of a bio_ctrl,
so move it in there instead of passing it down the deep call chain
all the way down to alloc_new_bio.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>